Blends of polymers as wet strengthening agents for paper

ABSTRACT

Resin systems and methods for making and using same are provided. The method for making a paper product can include contacting a plurality of pulp fibers with a resin system. The resin system can include a first polyamidoamine-epihalohydrin resin and a second resin that can include a second polyamidoamine-epihalohydrin resin, a urea-formaldehyde resin, or a mixture thereof to produce a paper product. The first resin and the second resin can be sequentially or simultaneously contacted with the plurality of pulp fibers. The period for sequential addition between the first resin and the second resin is about 1 second to about 1 hour.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 14/679,442, filed on Apr. 6, 2015, which is a divisional ofU.S. patent application Ser. No. 14/108,492, filed on Dec. 17, 2013, nowU.S. Pat. No. 8,999,110, which claims priority to U.S. ProvisionalPatent Application No. 61/739,329, filed on Dec. 19, 2012, which are allincorporated by reference herein.

FIELD

Embodiments described generally relate to paper strengthening agents.More particularly, such embodiments relate to wet strengthening agents.

BACKGROUND

Paper is sheet material containing interconnected small, discretefibers. The fibers are usually formed into a sheet on a fine screen froma dilute water suspension or slurry. Typically paper is made fromcellulose fibers, although occasionally synthetic fibers are used. Thewet strength of paper is defined (U.S. Pat. No. 5,585,456) as theresistance of the paper to rupture or disintegration when it is wettedwith water. Paper products made from untreated cellulose fibers losetheir strength rapidly when they become wet, i.e., they have very littlewet strength. Wet strength of ordinary paper is only about 5% of its drystrength. Various methods of treating paper products have been employedto overcome this disadvantage.

Wet strength resins applied to paper are either of the “permanent” or“temporary” type, which are defined by how long the paper retains itswet strength after immersion in water. While wet strength retention is adesirable characteristic in packaging materials, it presents a disposalproblem. Paper products having such characteristics are degradable onlyunder undesirably severe conditions. While some resins are known toimpart temporary wet strength and thus would be suitable for sanitary ordisposable paper uses, they often suffer from one or more drawbacks. Forexample, their wet strength is generally of a low magnitude (aboutone-half of the level achievable for permanent-type resins), they areeasily attacked by mold and slime, or they can only be prepared asdilute solutions.

There is a need, therefore, for improved methods for impartingappropriate levels of wet strength and/or repulpability to paperproducts.

SUMMARY

Resin systems and methods for making and using same are provided. In atleast one specific embodiment, the method for making a paper product caninclude contacting a plurality of pulp fibers with a resin system. Theresin system can include a first polyamidoamine-epihalohydrin resin anda second resin that can include a second polyamidoamine-epihalohydrinresin, a urea-formaldehyde resin, or a mixture thereof to produce apaper product. The first resin and the second resin can be sequentiallyor simultaneously contacted with the plurality of pulp fibers. Theperiod for sequential addition between the first resin and the secondresin can be about 1 second to about 1 hour.

In at least one specific embodiment, the paper product can include aplurality of pulp fibers and an at least partially cured resin system.The resin system, prior to curing, can include a firstpolyamidoamine-epihalohydrin resin and a second resin that can include asecond polyamidoamine-epihalohydrin resin, a urea-formaldehyde resin, ora mixture thereof. The first resin and the second resin can besequentially or simultaneously contacted with the plurality of pulpfibers. The period for sequential addition between the first resin andthe second resin can be about 1 second to about 1 hour.

In at least one specific embodiment, the composition can include aplurality of pulp fibers and a resin system. The resin system caninclude a first polyamidoamine-epihalohydrin resin and a second resinthat can include a second polyamidoamine-epihalohydrin resin, aurea-formaldehyde resin, or a mixture thereof. The composition can bemade by contacting first resin and the second resin sequentially orsimultaneously with the plurality of pulp fibers. The period forsequential addition between the first resin and the second resin can beabout 1 second to about 1 hour.

DETAILED DESCRIPTION

It has been surprisingly and unexpectedly discovered that mixing,blending, or otherwise combining two or more resins via sequential orsimultaneous addition, with respect to one another, to the pulp fiberscan provide resin systems with improved performance characteristics. Forexample, the resin systems can surprisingly and unexpectedly enhance thestrength of paper such as the wet strength of paper and/or therepulpability of the paper. In another example, the resin systems canexhibit faster cure rates. In another example, resin systems thatinclude the blend of two or more different resins that involve differentcuring mechanisms can surprisingly and unexpectedly show a synergeticeffect as strengthening agents for paper. In at least some embodiments,some inter-molecular reactions can be developed between the two or moreresins.

The resin system can be made by mixing, blending, stirring, contacting,or otherwise combining two or more resins or “component resins” with oneanother, where each resin or “component resin” has a different order ofaddition. In one embodiment, the first resin or the second resin can beadded sequentially or simultaneously to the pulp fibers. In one or moreembodiments, the resin system can include the first resin and the secondresin, and optionally any number of additional resins, e.g., a thirdresin, a fourth resin, a fifth resin, or more, where the period ofsequential addition of resins differ from one another yielding a resinsystem with improved properties.

The first resin can be present in the resin system in an amount of about0.1 wt % to about 99.9 wt %, based on the combined solids weight of thefirst resin and the second resin. For example, the first resin can bepresent in an amount from a low of about 0.5 wt %, about 1 wt %, about 5wt %, about 10 wt %, about 15 wt %, about 25 wt %, or about 35 wt % to ahigh of about 65 wt %, about 75 wt %, about 85 wt %, or about 95 wt %,based on the combined solids weight of the first and second resins. Inanother example, the first resin can be present from about 0.5 wt % toabout 10 wt %, about 10 wt % to about 20 wt %, about 20 wt % to about 30wt %, about 40 wt % to about 60 wt %, about 60 wt % to about 80 wt %,about 80 wt % to about 90 wt %, or about 90 wt % to about 99.5 wt %,based on the combined solids weight of the first and second resins. Inanother example, the first resin can be present from about 5 wt % toabout 25 wt %, about 20 wt % to about 45 wt %, about 30 wt % to about 55wt %, about 45 wt % to about 70 wt %, about 40 wt % to about 80 wt %, orabout 65 wt % to about 85 wt %, based on the combined solids weight ofthe first and second resins. The second resin can be present in anamount from a low of about 0.5 wt %, about 1 wt %, about 5 wt %, about10 wt %, about 15 wt %, about 25 wt %, or about 35 wt % to a high ofabout 65 wt %, about 75 wt %, about 85 wt %, or about 95 wt %, based onthe combined solids weight of the first and second resins. In anotherexample, the second resin can be present in an amount from about 0.5 wt% to about 10 wt %, about 10 wt % to about 20 wt %, about 20 wt % toabout 30 wt %, about 40 wt % to about 60 wt %, about 60 wt % to about 80wt %, about 80 wt % to about 90 wt %, or about 90 wt % to about 99.5 wt%, based on the combined solids weight of the first and second resins.In another example, the second resin can be present from about 5 wt % toabout 25 wt %, about 20 wt % to about 45 wt %, about 30 wt % to about 55wt %, about 45 wt % to about 70 wt %, about 40 wt % to about 80 wt %, orabout 65 wt % to about 85 wt %, based on the combined solids weight ofthe first and second resins.

When three or more resins are combined to provide the resin blend orresin system, the three or more resins can be present in any amount. Forexample, in the context of a resin system that includes the first resin,the second resin, and a third resin, the first resin can be present inan amount of from about 0.5 wt % to about 99 wt %, the second resin canbe present in an amount of from about 0.5 wt % to about 99 wt %, and thethird resin can be present in an amount of from about 0.5 wt % to about99 wt %, based on the combined solids weight of the first, second, andthird resins. For simplicity and ease of description, the resin systemwill be further discussed and described in the context of a two resinsystem or a “two component” resin system, i.e., as a resin blend havinga first resin and a second resin, combined with one another.

The resin system can be made by mixing, blending, stirring, contacting,or otherwise combining two or more resins with one another. The resinscan be a liquid or a solution of the resin. For example, the firstand/or second resins can be mixed, blended, stirred, contacted, orotherwise combined with one or more solvents. The solvent can be water,an organic solvent, or a combination thereof. For example, the resinsand/or the resin system can be a in a liquid phase or solution. In atleast one example, the resins and/or the resin system can be in the formof an aqueous solution.

Various different types of processes and/or reactor configurations canbe used to produce the resin system, including, but not limited to,series reactors (i.e., sequentially-configured reactors) and singlereactors. The resin system, for example, can be a reactor blend (alsosometimes referred to as a chemical blend). A reactor blend is a blendthat is formed (polymerized) in a single reactor. The resin system canalso be a physical blend, i.e., a composition formed by thepost-polymerization blending or mixing together of two or more resins,e.g., at least one high molecular weight resin and at least one lowmolecular weight resin, where each resin is polymerized using the sameor different catalyst systems.

Blending resins can be used to make a resin system having one or moreimproved properties relative to either the first resin, the secondresin, or a resin made to have the same or different molar ratio and/ormolecular weight distribution as the resin system, thus yielding a resinsystem that can be more suited to the requirements for a particularapplication. While not wishing to be bound by any particular theory, itis believed that the individual resins bring their unique chemical andphysical properties to the resin system. Also, the resins can producesynergistic effects with one another for certain properties withoutdetrimentally affecting other properties.

Many kinds of resins can be used make the resin system. For example, theresins can include, but are not limited to, one or morepolyamidoamine-epichlorohydrin (PAE) resins, one or moreurea-formaldehyde (UF) resins, or any mixture thereof. In one example,the first resin can be a polyamidoamine-epichlorohydrin (PAE) resin or aurea-formaldehyde (UF) resin and the second resin can be apolyamidoamine-epichlorohydrin (PAE) resin or a urea-formaldehyde (UF)resin. The first resin can be present in an amount of about 1 wt % toabout 99 wt %, based on the total weight the resin system. The secondresin can be present in an amount of about 1 wt % to about 99 wt %,based on the total weight the resin system. The first resin or thesecond resin can be added sequentially or simultaneously to the pulpfibers, where the period for sequential addition between the resins isfrom about 1 second to about 1 hour. The first resin or the second resincan be added sequentially to the pulp fibers where the period forsequential addition between the first and second resin is about 1 secondto about 1 hour. If the first resin and the second resin are bothpolyamidoamine-epichlorohydrin (PAE) resins, or urea-formaldehyderesins, or another resin the first and second resins can be differentfrom one another. For example, the first and second resins can havedifferent molecular weights, different structures, different molarratios of reactants, and/or other differences. Such resin systems can beused to enhance the strength of paper, particularly the wet strength ofpaper. In other examples, the resin system can include three or moreresins.

The viscosity of the resin system can vary widely. For example, theviscosity of the resin system can range from a low of about 1 centipoise(cP), about 100 cP, about 250 cP, about 500 cP, or about 700 cP to ahigh of about 1,000 cP, about 1,250 cP, about 1,500 cP, about 2,000 cP,or about 2,200 cP at a temperature of about 25° C. In another example,the resin system can have a viscosity from about 1 cP to about 125 cP,about 125 cP to about 275 cP, about 275 cP to about 525 cP, about 525 cPto about 725 cP, about 725 cP to about 1,100 cP, about 1,100 cP to about1,600 cP, about 1,600 cP to about 1,900 cP, or about 1,900 cP to about2,200 cP at a temperature of about 25° C. In another example, the resinsystem can have a viscosity from about 1 cP to about 45 cP, about 45 cPto about 125, about 125 cP to about 550 cP, about 550 cP to about 825cP, about 825 cP to about 1,100 cP, about 1,100 cP to about 1,600 cP, orabout 1,600 cP to about 2,200 cP at a temperature of about 25° C. Theviscosity can be measured using a Brookfield viscometer. For example,the Brookfield Viscometer can be equipped with a small sample adaptersuch a 10 mL adapter and the appropriate spindle to maximize torque suchas a spindle no. 31.

The resin system can have a pH from a low of about 1, about 2, about 3,about 4, about 5, about 6, about 7 to a high of about 8, about 9, about10, about 11, about 12, or about 13. In another example, resin systemcan have a pH from about 1 to about 2.5, about 2.5 to about 3.5, about3.5 to about 4.5, about 4.5 to about 5.5, about 5.5 to about 6.5, about6.5 to about 7.5, about 7.5 to about 8.5, about 8.5 to about 9.5, about9.5 to about 10.5, about 10.5 to about 11.5, about 11.5 to about 12.5,or about 12.5 to about 13.

The resin system, in addition to the first and second resins caninclude, but is not limited to, one or more other resins and/oradditives. For example, the one or more other resins or additives can becombined with the first resin and/or the second resin and/or thecombined first and second resins to produce the resin system.Illustrative additives can include, but are not limited to, waxes and/orother hydrophobic additives, water, filler material(s), extenders,surfactants, release agents, dyes, fire retardants, scavengers,biocides, or any combination thereof. Typical extenders can include, forexample, wheat flour. Other suitable extenders can include, but are notlimited to, polysaccharides, sulfonated lignins, and the like.Illustrative polysaccharides can include, but are not limited to,starch, cellulose, gums, such as guar and xanthan, alginates, pectin,gellan, or any combination thereof. Suitable polysaccharide starches caninclude, for example maize or corn, waxy maize, high amylose maize,potato, tapioca, and wheat starch. Other starches such as geneticallyengineered starches can include, but are not limited to, high amylosepotato and potato amylopectin starches.

If the resin system includes one or more additives, the amount of eachadditive can be from a low of about 0.01 wt % to a high of 50 wt %,based on the total weight of the resin system. For example, the amountof any given additive can range from a low of about 0.01 wt %, about0.05 wt %, about 0.1 wt %, about 0.5 wt %, or about 1 wt % to a high ofabout 3 wt %, about 5 wt %, about 7 wt %, or about 9 wt %, based on thetotal weight of the resin system. In another example, the amount of anygiven additive can be from a low of about 1 wt %, about 5 wt %, about 10wt %, about 15 wt %, or about 20 wt % to a high of about 25 wt %, about30 wt %, about 35 wt %, about 40 wt %, or about 45 wt %, based on thetotal weight of the resin system.

As noted above, the resin system can include one or morepolyamidoamine-epichlorohydrin (PAE) resins. A variety of techniques areknown in the art for making polyamidoamine-epichlorohydrin (PAE) resinscan be employed. The polyamidoamine-epichlorohydrin (PAE) resin can beproduced via any suitable process. For example, conventional PAE resinsthat can provide permanent wet strength to paper can be obtained bymodifying polyamidoamine polymers or prepolymers such as thepolyamidoamine prepolymer (A) by reaction with epichlorohydrin (B)(“epi”) to form a polyamidoamine-epichlorohydrin (PAE) resin.

Conventional resin syntheses capitalize on the difunctional nature ofepichlorohydrin to use the epoxy and chlorine groups for bothcross-linking and generation of quaternary nitrogen sites. In theseconventional syntheses, the asymmetric functionality of epichlorohydrinleads to ring opening upon reaction of its epoxy group with secondaryamines, followed by the pendant chlorohydrin moiety eitherintra-molecularly cyclizing to generate azetidinium functionality orinter-molecularly (cross-linking) with another polyamidoamine molecule.Thus, the first step of reacting polyamidoamine prepolymer A with epi Boccurs with ring-opening of the epoxy group by secondary amine groups ofthe prepolymer backbone at relatively low temperature. Newfunctionalized polymer C having chlorohydrin pendant groups isgenerated, and this process typically results in little or nosignificant change in the prepolymer molecular weight.

The second step involves two competing reactions of the pendantchlorohydrin groups: (1) an intramolecular cyclization which generates acationic azetidinium chloride functionality, in which no increase inmolecular weight is observed; and (2) an intermolecular alkylationreaction to cross-link the polymer, which significantly increases itsmolecular weight. The results of both reactions can be as illustrated inthe PAE-epichlorohydrin resin structure D below. In practice, thealkylation of epichlorohydrin, the intra-molecular cyclization and thecross-linking reactions can occur simultaneously, but at differentrates.

The finished wet strength polymer product can contain a small amount ofresidual pendant chlorohydrin as illustrated in structure D, and a3-carbon cross-linked group with 2-hydroxyl functionality, with a fairlylarge amount of quaternary azetidinium chloride functionality. Theproduct also can contain substantial amounts of the epichlorohydrinhydrolysis products 1,3-DCP, and 3-CPD.

The relative rates of the three main reactions in this conventionalmethod, namely the pendant chlorohydrin formation (ring opening),cyclization to azetidinium ion groups (cationization), and cross-linking(intermolecular alkylation), generally approximate a rate of about140:4:1, respectively, when carried out at room temperature. Therefore,the pendant chlorohydrin groups form very quickly from ring openingreaction of the epichlorohydrin epoxide and the secondary amine in theprepolymer. This first step is performed at lower temperature (forexample, around 25-30° C.).

In the second step, the chlorohydrin groups then relatively slowlycyclize to form cationic azetidinium groups. Even more slowly,cross-linking occurs, for example, by: (1) a tertiary amine, forexample, of a chlorohydrin pendent group reacting with moiety secondaryamine; and/or (2) intermolecular alkylation of a tertiary amine with apendant chlorohydrin moiety.

In order to maintain practical utility for minimum reaction cycle times,the conventional manufacturing process typically heat the reactionmixture to increase the reaction rates, for example to about 60° C. toabout 70° C. The reactions can also be carried out at high solidscontent in order to maximize or increase reactor throughput and toprovide finished wet strength resins at the highest solids possible tominimize shipping costs. High concentration favors the slower,inter-molecular reaction. Under these high temperature and highconcentration conditions, the reaction rates between intramolecularcyclization and cross-linking become competitive. Thus, one problemencountered in the conventional manufacturing process is that thecross-linking reaction rate becomes fast enough that the desiredviscosity end-point (molecular weight) is achieved at the expense ofazetidinium ion group formation. If the reaction was allowed to continuebeyond the desired viscosity end-point in order to generate higherlevels of azetidinium groups, the reaction mixture would likely gel andform a solid mass.

Since both high azetidinium group content and high molecular weights canbe useful for maximum wet strength efficiency of PAE resins, azetidiniumgroup formation and cross-linking can be maximized or increased withoutgelling the product or providing a product that gels during storage.These conditions, coupled with the desire for high solids to minimizeshipping costs, have been limiting aspects of the formation of higherefficiency wet strength resin products.

In other embodiments, using new functionally-symmetrical (“symmetrical”)cross-linkers and mono-functional modifiers and separating into discretesteps the reaction of prepolymer with new cross-linkers from thereaction of intermediate cross-linked prepolymer with epichlorohydrin,new, non-conventional PAE resins with enhanced properties and/orimproved flexibility in their synthesis are provided. In addition toproviding generally improved wet tensile development over currenttechnologies, the products and methods can provide higher azetidiniumion content, additional degrees of reactive functionalization, maximizedmolecular weight, and good storage stability. Moreover, the wet strengthproducts can have substantially reduced levels of 1,3-DCP and 3-CPDwhich typically accompany epichlorohydrin wet strength resin synthesis.

Wet strength resins can be obtained by modifying amine-containingpolymers (polyamine polymers) such as polyamine, polyamidoamine,polyethyleneimine (PEI), polyvinyl amine, and the like. Modifyingamine-containing polymers can, for example, add more cationic chargesand/or reactive groups and/or increase their molecular weight.

In one embodiment, the polyamine, which may be referred to herein as apolyamine prepolymer, can have the following structure:

where R can be an alkyl, a hydroxyalkyl, an amine, an amide, an aryl, aheteroaryl, or a cycloalkyl. In structure P, w can be an integer from 1to about 10,000. As provided in the definitions section, the R groupssuch as “alkyl” or “hydroxyalkyl” are intended to provide a convenientdescription in which the conventional rules of chemical valence apply;therefore, R of structure P may be described as alkyl or hydroxyalkyl,which is intended to reflect the “R” group is divalent and mayalternatively be described as a hydroxyalkylene.

The most widely used and most effective wet strength resin products aregenerally derived from polyamidoamine prepolymers reacted withepichlorohydrin, to form so-called polyamidoamine-epichlorohydrin (PAE)resins. Therefore, when polyamidoamines are used to exemplify the aprocess or resin disclosed herein, it is intended that the disclosure,process, and resin are not limited to polyamidoamine-based systems, butare applicable to any amine-containing polymer (polyamine) such asstructure P and other amine-containing polymers.

Epichlorohydrin is a difunctional compound having different, hence“asymmetric”, chemical functionalities, epoxy and chlorine groups. Thisasymmetric functionality allows the epichlorohydrin ring to open uponreaction with the epoxy group with secondary amines, followed by thependant chlorohydrin moieties used for both: (1) intramolecularcyclization to generate a cationic azetidinium functionality; or 2)intermolecular cross-linking the polymer to increase molecular weight.Epichlorohydrin resin structure D illustrates the result of bothreactions in a polyamidoamine-epichlorohydrin (PAE) resin.

Discussed and described herein are formulations and processes for makingnew, non-conventional PAE resins with increased levels of cationiccharge from enhanced azetidinium ion content (greater charge density),additional functionality, optimized or maximized molecular weights, highsolids contents, and/or lower concentrations of DCP and/or CPD. In anaspect, the disclosed method separates the resin synthesis into twoseparate and controllable steps. The first constructs an intermediatemolecular weight, cross-linked prepolymer, prepared upon reacting thePAE prepolymer with a functionally-symmetric cross-linker. Unlike thefunction of the asymmetric cross-linker epichlorohydrin, the symmetriccross-linkers of this disclosure utilize the same moiety for reactionwith both prepolymer secondary amine groups to effect cross-linking. Ifdesired, monofunctional groups can be used before, after, or during thecross-linking step to impart additional functionality to a prepolymerwithout the cross-linking function. The second step utilizesepichlorohydrin to impart cationic functionality without it beingrequired for any cross-linking function, by using a reduced amount ofepichlorohydrin to maximize azetidinium ion formation on the polymer.This new, non-conventional process stands in contrast to conventionalpractice which is limited by the need to optimize competing azetidiniumion formation and cross-linking mechanisms that occur simultaneously.

Polyamine Prepolymer

A range of polyamines (polyamine prepolymers) can be used as a precursorto the wet strength resins disclosed herein. The polyamine prepolymerscan include primary and/or secondary amine moieties that are linked withat least one spacer. By way of example, in one aspect, the polyamine,which can be referred to herein as a polyamine prepolymer, can have thefollowing structure:

where R can be, for example, an alkyl, a hydroxyalkyl, an amine, anamide, an aryl, heteroaryl, or a cycloalkyl. In structure P, w can be aninteger from 1 to about 10,000; alternatively, from 1 to about 5,000;alternatively, from 1 to about 3,000; alternatively, from 1 to about1,000; alternatively, from 1 to about 100; or alternatively, from 1 toabout 10. These “R” groups, for example “alkyl”, are intended to providea convenient description of the specified groups that are derived fromformally removing one or more hydrogen atoms (as needed for theparticular group) from the parent group. Therefore, the term “alkyl” instructure P would apply the conventional rules of chemical valence, butwould include, for example, an “alkanediyl group” which is formed byformally removing two hydrogen atoms from an alkane (either two hydrogenatoms from one carbon atom or one hydrogen atom from two differentcarbon atoms). Such an alkyl group can be substituted or unsubstitutedgroups, can be acyclic or cyclic groups, and/or may be linear orbranched unless otherwise specified. A “hydroxyalkyl” group includes oneor more hydroxyl (OH) moieties substituted on the “alkyl” as defined.

In this aspect and unless otherwise indicated, alkyl R of structure Pcan be an alkyl moiety that is linear (straight chain) or branched.Moiety R can also be a cycloalkyl, that is, a cyclic hydrocarbon moietyhaving from 1 to about 25 carbon atoms. For example, R can have from 1to 25, from 1 to 20, from 1 to 15, from 1 to 12, from 1 to 10, from 1 to8, from 1 to 6, or from 1 to 4 carbon atoms. Also by way of example, Rcan have from 2 to 10, 2 to 8, 2 to 6, or 2 to 4 carbon atoms. In afurther aspect, R can be a C₁ moiety, a C₂ moiety, a C₃ moiety, a C₄moiety, a C₅ moiety, a C₆ moiety, a C₇ moiety, a C₈ moiety, a C₉ moiety,a C₁₀ moiety, a C₁₁ moiety, a C₁₂ moiety, a C₁₃ moiety, a C₁₄ moiety, aC₁₅ moiety, a C₁₆ moiety, a C₁₇ moiety, a C₁₈ moiety, a C₁₉ moiety, aC₂₀ moiety, a C₂₁ moiety, a C₂₂ moiety, a C₂₃ moiety, a C₂₄ moiety, aC₂₅ moiety, a C₂₆ moiety, a C₂₇ moiety, a C₂₈ moiety, a C₂₉ moiety, aC₃₀ moiety.

In the polyamine prepolymer structure P illustrated supra, R also can bea poly-primary amine, such as polyvinyl amine and its copolymers.Examples of a poly-primary amine that can constitute R in structure Pinclude, but are not limited to the following structures, as well ascopolymers with olefins and other unsaturated moieties, where n can bean integer from 1 to about 25:

Alternatively, n can be an integer from 1 to about 20; alternatively,from 1 to about 15; alternatively, from 1 to about 12; alternatively,from 1 to about 10; or alternatively, from 1 to about 5. In anotheraspect, n can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,17, 18, 19, 20, 21, 22, 23, 24, or 25.

Suitable polyamines (polyamine prepolymers) for use in preparing resinsdiscussed and described herein include, but are not limited to,polyalkylene polyamines, such as polyethylenepolyamines includingdiethylenetriamine (DETA), triethylenetetramine (TETA), aminoethylpiperazine, tetraethylenepentamine, pentaethylenehexamine,N-(2-aminoethyl)piperazine, N,N-bis(2-aminoethyl)-ethylenediamine,diaminoethyl triaminoethylamine, piperazinethyl triethylenetetramine,and the like. Also useful in preparing polyamine prepolymers for use inthe resin preparations of this disclosure include, ethylene diamine, lowmolecular weight polyamidoamines, polyvinylamines, polyethyleneimine(PEI) and copolymers of vinyl amine with other unsaturatedco-polymerizable monomers such as vinyl acetate and vinyl alcohol.

According to an aspect of polyamine prepolymer P, w is a number rangecorresponding to the polyamine prepolymer weight average molecularweight (Mw) from about 2,000 to about 1,000,000. The Mw of polyamineprepolymer P can also can be from about 5,000 to about 750,000;alternatively, from about 7,500 to about 500,000; alternatively, fromabout 10,000 to about 200,000; alternatively, from about 20,000 to about150,000; or alternatively, from about 30,000 to about 100,000.

Polyamidoamine Prepolymer

A range of polyamidoamine prepolymers also can be used as a precursor tothe wet strength resins discussed and described herein. Thepolyamidoamine prepolymers can be made by the reaction of a polyalkylenepolyamine having at least two primary amine groups and at least onesecondary amine group with a dicarboxylic acid, in a process to form along chain polyamide containing the recurring groups as disclosedherein. In one aspect, the polyamidoamine prepolymer can have thefollowing structure (X):

where R¹ is (CH₂)m, where m is 2, 3, 4, or 5; R² is (CH₂)n, where n is2, 3, or 4; w is 1, 2, or 3; and p is a number range corresponding tothe polyamidoamine prepolymer Mw from about 2,000 to about 1,000,000.The Mw also can be from about 5,000 to about 100,000; alternatively,from about 7,500 to about 80,000; alternatively, from about 10,000 toabout 60,000; alternatively, from about 20,000 to about 55,000; oralternatively, from about 30,000 to about 50,000.

In an aspect, the polyamidoamine prepolymer can have the followingstructure (Y):

where R³ is (CH₂)q, where q is from 0 to 40; and r is a number rangecorresponding to the polyamidoamine prepolymer Mw from about 2,000 toabout 1,000,000. Similarly, the Mw also can be from about 5,000 to about100,000; alternatively, from about 7,500 to about 80,000; alternatively,from about 10,000 to about 60,000; alternatively, from about 20,000 toabout 55,000; or alternatively, from about 30,000 to about 50,000. Thus,in the structure (CH₂)q, q can also range from 0 to about 40;alternatively, from 0 to about 35; alternatively, from 0 to about 30;alternatively, from 0 to about 25; alternatively, from 0 to about 20;alternatively, from 0 to about 15; alternatively, from 0 to about 12;alternatively from 1 to about 40, alternatively from 1 to about 35,alternatively from 1 to about 30, alternatively from 1 to about 25,alternatively from 1 to about 20, alternatively from 1 to about 15,alternatively, from 1 to about 12; alternatively, from 1 to about 10;alternatively, from 1 to about 8; or alternatively, from 1 to about 6.

In a further aspect, the polyamidoamine prepolymer also may have thefollowing structure (Z):

—[—NH(C_(n)H_(2n)—NH)_(p)—CO—(CH₂)_(m)—CO—]—   (Z),

where n is 1 to 8; p is 2 to 5; and m is 0 to 40, and molecular weightranges similar to those of formula (X) and (Y) apply. For example, theMw can be from about 2,000 to about 1,000,000. The Mw also can be fromabout 5,000 to about 100,000; alternatively, from about 7,500 to about80,000; alternatively, from about 10,000 to about 60,000; alternatively,from about 20,000 to about 55,000; or alternatively, from about 30,000to about 50,000

As disclosed, suitable polyamidoamines can be prepared by reacting adicarboxylic acid (diacid), or a corresponding dicarboxylic acid halideor diester thereof, with a polyamine such as a polyalkylene polyamine.Suitable polyamines include those polyamines (polyamine prepolymers)disclosed herein that can be used as precursors for the wet strengthresins themselves. For example, the polyamidoamine can be made byreacting one or more polyalkylene polyamines, such aspolyethylenepolyamines including ethylenediamine itself,Diethylenetriamine (DETA), triethylenetetramine (TETA), aminoethylpiperazine, tetraethylenepentamine, pentaethylenehexamine,N-(2-aminoethyl)piperazine, N,N-bis(2-aminoethyl)-ethylenediamine,diaminoethyl triaminoethylamine, piperazinethyl triethylenetetramine,and the like, with one or more polycarboxylic acids such as succinic,glutaric, 2-methylsuccinic, adipic, pimelic, suberic, azelaic, sebacic,undecanedioic, dodecandioic, 2-methylglutaric, 3,3-dimethylglutaric andtricarboxypentanes such as 4-carboxypimelic; alicyclic saturated acidssuch as 1,2-cyclohexanedicarboxylic, 1-3-cyclohexanedicarboxylic,1,4-cyclohexanedicarboxylic and 1-3-cyclopentanedicarboxylic;unsaturated aliphatic acids such as maleic, fumaric, itaconic,citraconic, mesaconic, aconitic and hexane-3-diotic; unsaturatedalicyclic acids such as Δ4-cyclohexenedicarboxylic; aromatic acids suchas phthalic, isophtalic, terephthalic, 2,3-naphthalenedicarboxylic,benzene-1,4-diacetic, and heteroaliphatic acids such as diglycolic,thiodiglycolic, dithiodiglycolic, iminodiacetic and methyliminodiacetic.In one embodiment, diacids and their related diesters of the formulaRO₂C(CH₂)_(n)CO₂R (where n=1 to 10 and R=H, methyl, or ethyl), andmixtures thereof can be used. Adipic acid is readily available and isoften used.

Symmetric Cross-Linker

Generally, the secondary amines of the polyamine prepolymers can bereacted with one or more symmetrical cross-linkers. In an aspect, thisreaction can provide for a greater degree of control over thecross-linking process. This reaction can also provide an intermediatecross-linked prepolymer with a higher molecular weight than the startingprepolymer. The viscosity end-point and thus the molecular weight of theintermediate can be easily pre-determined and controlled simply by theamount of symmetrical cross-linker employed. The cross-linking reactioncan proceed to an end-point as the cross-linker is consumed and stopwhen consumption of cross-linker is complete. A decreased andmeasureable amount of secondary amine functionality can remain availablefor further functionalization.

In this cross-linking step, the polyamine prepolymer can be reacted witha deficiency of the symmetric cross-linker, based on the total amount ofsecondary amines available for cross-linking, to provide a partiallycross-linked polyamine prepolymer. Thus, the partially cross-linkedpolyamine prepolymer can have a higher molecular weight than thepolyamine prepolymer, even though it is an intermediate in the processand it retains a portion of the secondary amine groups present in thepolyamine prepolymer. In a further aspect, the partially cross-linkedprepolymer can retain a majority of the secondary amine groups presentin the polyamine prepolymer, because less than 50% of the stoichiometryamount of symmetric cross-linker generally is used.

Based on the prepolymer repeating unit having a single secondary aminesubject to reaction, and the symmetric cross-linker having two reactivemoieties, a stoichiometric reaction of prepolymer to cross-linkerrequires 2:1 molar ratio, and practically, a 2:1 or higher molar ratioof prepolymer to cross-linker is utilized. In one aspect, the symmetriccross-linker to prepolymer molar ratios can be selected to provide morethan 0%, but less than 50%, less than 45%, less than 40%, less than 35%,less than 30%, less than 25%, less than 20%, less than 15%, less than10%, less than 5%, less than 4%, less than 3%, less than 2%, less than1%, less than 0.75%, or less than 0.5% of the stoichiometric ratio ofcross-linker to prepolymer. These values reflect the combined molaramounts when using more than one symmetric cross-linker.

Examples of symmetric cross-linkers include, but are not limited to, adi-acrylate, a bis(acrylamide), a di-epoxide, and a polyazetidiniumcompound. By way of example, useful symmetric cross-linkers can beselected from or can comprise, the following:

where R⁴ is (CH₂)_(t) and t is 1, 2, or 3;

where x is from about 1 to about 100;

where y is from about 1 to about 100;

where x′+y′ is from about 1 to about 100; and/or

where z is from about 1 to about 100; including any combination thereof.

Specific examples of symmetric cross-linkers can be selected from, oralternatively can include, N,N′-methylene-bis-acrylamide,N,N′-methylene-bis-methacrylamide, poly(ethylene glycol) diglycidylether, poly(propylene glycol) diglycidyl ether, polyethylene glycoldiacrylate, polyazetidinium compounds, and any combination thereof.

In accordance with a further aspect, the symmetric cross-linker can beselected from or can include certain polymers or co-polymers that have atype of functional moiety that is reactive with secondary amines, thatis, that can function as a symmetrical cross-linker according to thisdisclosure. In one aspect, these polymeric symmetric cross-linkers canbe polymers or copolymers that comprise azetidinium functional groups.These polymeric symmetric cross-linkers can be, for example, copolymersof acrylates, methacrylates, alkenes, dienes, and the like, withazetidinium-functionalized monomers such as1-isopropyl-3-(methacryloyloxy)-1-methylazetidinium chloride Q or1,1-diallyl-3-hydroxyazetidinium chloride R, the structures of which areillustrated.

The polymeric symmetric cross-linkers also can be or can include, forexample, copolymers of acrylates, methacrylates, alkenes, dienes, andthe like, with other azetidinium ˜functionalized monomers such ascompounds S, T, or U, as shown here.

In another embodiment, the symmetric cross-linker can be selected fromor can include a copolymer of an acrylate, a methacrylate, an alkene, ora diene, with an azetidinium-functionalized monomer selected from Q, R,S, T, U, and a combination thereof, where the fraction ofazetidinium-functionalized monomer to acrylate, methacrylate, alkene, ordiene monomer in the copolymer can be from about 0.1% to about 12%. In afurther aspect, the fraction of azetidinium-functionalized monomer toacrylate, methacrylate, alkene, or diene monomer in the copolymer can befrom about 0.2% to about 10%; alternatively, from about 0.2% to about10%; alternatively, from about 0.5% to about 8%; alternatively, fromabout 0.75% to about 6%; or alternatively, from about 1% to about 5%.Examples of these types of symmetric cross-linker polymers andco-polymers can be found in the following references, each of which isincorporated herein by reference in pertinent part: Y. Bogaert, E.Goethals and E. Schacht, Makromol. Chem., 182, 2687-2693 (1981); M.Coskun, H. Erten, K. Demirelli and M. Ahmedzade, Polym. Degrad. Stab.,69, 245-249 (2000); and U.S. Pat. No. 5,510,004.

In other embodiment, the symmetric cross-linker can be selected from orcan include a minimally azetidinium-functionalized polyamidoamine. Thatis, the polyamidoamine can have minimal azetidinium functionalization,which is the reactive moiety in this type of symmetric cross-linker. Inthis case, the cross-linking function can be effected by the azetidiniummoieties, which can react with secondary amines of the polyamidoamineprepolymer. Polyamidoamines that can be used to prepare the minimallyazetidinium-functionalized polyamidoamines can have the same generalstructures and formulas that can be used for the preparation of theresin itself, such as structures X, Y, and Z illustrated herein. Anexample of a minimally azetidinium-functionalized polyamidoaminesuitable for use as a symmetric cross-linker is illustrated in thefollowing structure:

where p≧2 the q/p ratio is from about 10 to about 1000, and where thestructure includes at least two azetidinium moieties that function tocross-link, and that qualify a structure such as X as afunctionally-symmetrical cross-linker. As the q/p ratio indicates, thereis a small fraction of azetidinium moieties as compared to acid andamine residues. Moreover, the polyamidoamine X also can have thestructure where the q/p ratio is from about 12 to about 500;alternatively, from about 14 to about 400; alternatively, from about 16to about 300; alternatively, from about 18 to about 200; oralternatively, from about 20 to about 100. One type of minimallyazetidinium-functionalized polyamidoamine is provided in, for example,U.S. Pat. No. 6,277,242.

As illustrated by the molar ratios of the symmetric cross-linker to thePAE prepolymer, generally, a relatively small fraction of the availablesecondary amine sites can be subject to cross-linking to form thebranched or partially cross-linked polyamidoamine prepolymer. Inaddition to the molar ratios provided herein, for example, the symmetriccross-linker to prepolymer molar ratios can be selected to provide from0.01% to 5% of the stoichiometric ratio of cross-linker to prepolymer.In a further aspect, the symmetric cross-linker to prepolymer molarratios can provide from 0.1% to 4%; alternatively, from 0.2% to 3.5%;alternatively, from 0.3% to 3%; alternatively, from 0.4% to 2.5%;alternatively, from 0.5% to 2%; or alternatively, from 0.6% to 1.5% ofthe stoichiometric ratio of cross-linker to prepolymer. These valuesreflect the combined molar amounts when using more than one symmetriccross-linker.

By way of example, using a polyamidoamine prepolymer derived from adipicacid and diethylenetriamine (DETA) as an example, and cross-linking theprepolymer using methylene-bis-acrylamide (MBA), the partiallycross-linked polyamidoamine prepolymer can be illustrated by thefollowing structure:

where the R^(x) bridging moiety has the structure:

This illustration does not reflect the use of any mono-functionalmodifiers (infra) in addition to the symmetrical cross-linker.

Mono-Functional Modifier

The secondary amine groups of the polyamine prepolymers can also bereacted with one or more mono-functional compounds to impart any desiredchemical functionality to the prepolymer. The mono-functional compoundshave a reactive group that can react with secondary or primary amine anda non-reactive part which can be cationic (to increase the cationiccharge density), hydrophilic or hydrophobic (to adjust the interactionwith non-ionic segments of the cellulose fibers). As desired, thepolyamine prepolymer can be reacted with a deficiency of amono-functional modifier comprising one secondary amine-reactive moietyeither before, during, or after, the step of reacting the polyamineprepolymer with a deficiency of the symmetric cross-linker. Further, thereaction with a stoichiometric deficiency of a mono-functional modifiercan also be carried using any combination of reaction or additionbefore, during, or after, reaction with the symmetric cross-linker.

In one embodiment, the mono-functional modifier can be selected from orcan include a neutral or cationic acrylate compound, a neutral orcationic acrylamide compound, an acrylonitrile compound, a mono-epoxidecompound, or any combination thereof. According to a further aspect, themono-functional modifier can be selected from or can include an alkylacrylate, acrylamide, an alkyl acrylamide, a dialkyl acrylamide,acrylonitrile, a 2-alkyl oxirane, a 2-(allyloxyalkyl)oxirane, ahydroxyalkyl acrylate, an ω-(acryloyloxy)-alkyltrimethylammoniumcompound, an ω-(acrylamido)-alkyltrimethylammonium compound, and anycombination thereof. Examples of mono-functional modifiers areillustrated below.

In other embodiments, the mono-functional modifier can be selected fromor alternatively can include at least one of: methyl acrylate; alkylacrylate; acrylamide; N-methylacrylamide; N,N-dimethylacrylamide;acrylonitrile; 2-methyloxirane; 2-ethyloxirane; 2-propyloxirane;2-(allyloxymethyl)oxirane; 2-hydroxyethyl acrylate;2-(2-hydroxyethoxy)ethyl acrylate;2-(acryloyloxy)-N,N,N-trimethylethanaminium;3-(acryloyloxy)-N,N,N-trimethylpropan-1-aminium;2-acrylamido-N,N,N-trimethylethanaminium; 3-acrylamido-N,N,N-trimethylpropan-1-aminium; and1-isopropyl-3-(methacryloyloxy)-1-methylazetidinium chloride. Depending,at least in part, on the structure of the modifier, it can be seen thatupon reaction of these compounds with a secondary or primary amine, theportion that is non-reactive toward the amine can impart cationic chargeto assist in increasing the cationic charge density, can alter thehydrophilic or hydrophobic characteristics, for example to adjust theinteraction with non-ionic segments of the cellulose fibers, and/or canaffect other properties of the resulting intermediate cross-linkedprepolymer.

Halohydrin-Functionalized Polymer and Intramolecular Cyclization

Generally, by separating into discrete steps the reaction of thepolyamine prepolymer with the cross-linkers from the reaction of theintermediate cross-linked prepolymer with the epichlorohydrin, thesecond reaction step requires less epichlorohydrin than conventionalmethods to reach the desired end-point. Further, this second reactionstep can be effected under reaction conditions that favor optimizedazetidinium group formation over further cross-linking. The asymmetricfunctionality of epichlorohydrin is useful in this functionalization toallow a relatively facile reaction of the epoxy group with secondaryamines to form a pendant chlorohydrin moiety, followed by anintramolecularly cyclization of the pendant chlorohydrin to generate acationic azetidinium functionality. This latter intramolecularcyclization typically utilizes heating of the halohydrin-functionalizedpolymer.

In one embodiment, the second reaction step can be carried out using anyepihalohydrin, such as epichlorohydrin, epibromohydrin, andepiiodohydrin, or any combination thereof. However, epichlorohydrin istypically the most common epihalohydrin used in this reaction step. Whenreciting epichlorohydrin in this disclosure, such as in structures orreaction schemes, it is understood that any one or any combination ofthe epihalohydrins can be used in the process.

By way of example, using the partially cross-linked polyamidoamineprepolymer illustrated supra that was derived from adipic acid and DETAand cross-linking using MBA, the epichlorohydrin functionalizationproduct can illustrated by the following structure, termed a“halohydrin-functionalized polymer”.

As before, this illustration does not reflect the use of anymono-functional modifiers in addition to the symmetrical cross-linker.

The reaction of epihalohydrins such as epichlorohydrin is generallytailored to consume a high percentage or the remaining secondary aminemoieties in generating the halohydrin-functionalized polymer, in thiscase, a chlorohydrin-functionalized polymer.

The formation of the halohydrin-functionalized polymer can be carriedout using a range of epichlorohydrin molar ratios, but this reaction istypically carried out using an excess of epichlorohydrin. Thestoichiometric reaction of epichlorohydrin with a secondary amine grouprequires a 1:1 molar ratio of epichlorohydrin with a secondary amine. Inan aspect, from about 0.8 mole to about 3 moles of epichlorohydrin permole of secondary amine can be used. Alternatively, from about 0.9 moleto about 2.5 moles of epichlorohydrin per mole of secondary amine;alternatively, from about 1.0 mole to about 2.0 moles; alternatively,from about 1.1 mole to about 1.7 moles; alternatively, from about 1.2mole to about 1.5 moles; alternatively, from about 1.25 mole to about1.45 moles of epichlorohydrin per mole of secondary amine can be used.For example, the moles of epichlorohydrin per mole of secondary aminecan be about 0.8 moles, about 0.9 moles, about 1.0 moles, about 1.1moles, about 1.2 moles, about 1.3 moles, about 1.4 moles, about 1.5moles, or about 1.6 moles epichlorohydrin per mole of secondary amine.

A further aspect of the process can be that sufficient amounts ofsymmetric cross-linker and epihalohydrin can be employed such that theresin composition prepared by the process can include substantially nosecondary amine groups. This result can be effected by using the molaramounts and ratios disclosed herein, but resin compositions prepared bythis disclosure can include substantially no secondary amine groups evenwhen molar amounts and ratios outside those recited may be used. Bysubstantially no secondary amine groups, it is intended that less than10% of the original secondary amines in the starting PAE resin prior toit cross-linking, functionalization, and cationization reactions remain.Alternatively, less than 5%; alternatively, less than 2%; alternatively,less than 1%; alternatively, less than 0.5%; alternatively, less than0.2%; alternatively, less than 0.1%; alternatively, less than 0.01%;alternatively, less than 0.005%; or alternatively, less than 0.001% ofthe original secondary amines in the starting PAE resin remain.

The halohydrin (typically chlorohydrin)-functionalized polymer can beconverted to the wet-strength resin composition by subjecting it tocyclization conditions to form azetidinium ions. This step can includeheating the chlorohydrin-functionalized polymer. In contrast to theconventional method in which heating induces both cross-linking andcyclization, the cross-linking portion of this process is complete whenthe cyclization is carried out, thereby affording greater processcontrol and the ability to more closely tailor the desired properties ofthe resulting resin. Also in contrast to the conventional method, theprocess of this disclosure reduces and/or minimizes the formation of theepichlorohydrin by-products 1,3-dichloro-2-propanol (1,3-DCP or “DCP”)and 3-chloropropane-1,2-diol (3-CPD or “CPD”) remaining in the resin canbe reduced or minimized.

According to one aspect of the disclosure, the concentration ofepichlorohydrin 1,3-dichloro-2-propanol (1,3-DCP) remaining in the wetstrength resin at 25% solids (DCP @ 25%) can be less than about 15,000ppm; alternatively, less than about 14,000 ppm; alternatively, less thanabout 13,000 ppm; alternatively, less than about 12,000 ppm;alternatively, less than about 11,500 ppm; alternatively, less thanabout 11,000 ppm; alternatively, less than about 10,500 ppm;alternatively, less than about 10,000 ppm; alternatively, less thanabout 8,000 ppm; alternatively, less than about 6,000 ppm; oralternatively, less than about 5,000 ppm.

The following resin structure Z illustrates the result of thecyclization step to form the quaternary nitrogen (“cationization”) basedon the chlorohydrin-functionalized polymer Y shown supra, which has beensubjected to conditions sufficient to intramolecularly cyclize thependant chlorohydrin to impart azetidinium functionality.

In the process for forming the new, non-conventional PAE resin, the PAEresin is generated by subjecting the halohydrin-functionalized polymerto cyclization conditions sufficient to convert the halohydrin groups toform azetidinium ions. In one aspect, at least a portion of thehalohydrin groups can be cyclized to form azetidinium ions. According toa further aspect, at least 90% of the halohydrin groups can be cyclizedto form azetidinium ions. Alternatively, at least 95%; alternatively, atleast 97%; alternatively, at least 98%; alternatively, at least 98.5%;alternatively, at least 99%; alternatively, at least 99.5%;alternatively, at least 99.7%; alternatively, at least 99.8%; oralternatively, at least 99.9% of the halohydrin groups can be cyclizedto form azetidinium ions. In another aspect, about 90% or more, about91% or more, about 92% or more, about 93% or more, about 94% or more,about 95% or more, about 96% or more, about 97% or more, about 98% ormore, about 99% or more, about 99.3% or more, about 99.5% or more, about99.7% or more, about 99.9% or more of the halohydrin croups can becyclized to form azetidinium ions.

The amount of the halohydrin groups cyclized to form azetidinium ionscan be measured via titration with silver nitrate. More particularly,the total chlorine content for a first sample of a PAE resin can bemeasured by refluxing in the presence of potassium hydroxide to convertall covalently-bound chlorine to chloride ion, neutralizing with nitricacid, and titrating with a silver nitrate solution. The total chlorineis therefore the sum of covalently-bound chlorine and ionic chlorine.The amount of the ionic chloride is measured on a second sample of thePAE resin, which does not involve refluxing in the presence of thepotassium hydroxide. The total amount of chlorine minus the amount ofionic chloride is the amount of chlorine (halohydrin groups) that can becyclized to form azetidinium ions.

Additional steps in the new, non-conventional PAE resin processing canbe used, for example, to adjust the solids content of the PAE resin,beyond those described in detail above. For example, the resin can begenerated by converting the halohydrin-functionalized polymer to anazetidinium functionalized polymer. Following this step, the polymercomposition can be adjusted by pH such that the pH of the resin can befrom about pH 2 to about pH 4.5. Alternatively, the pH of the resin canbe from about pH 2.2 to about pH 4.2; alternatively, from about pH 2.5to about pH 4; or alternatively, from about pH 2.7 to about pH 3.7. ThispH adjustment step also may be followed by the step of adjusting thesolids content of the composition from about 10% to about 50% to formthe wet strength resin. Alternatively, the solids content of the resincan be adjusted from about 15% to about 40% or alternatively from about20% to about 30% to form the polyamidoamine-epihalohydrin resin. In oneaspect, the polyamidoamine-epihalohydrin resin can have a solids contentof about 25%.

The polyamidoamine-epihalohydrin resin can have a charge density that isenhanced over that of conventional resins. For example, the PAE resincan have a charge density of about 2 to about 4 mEq/g of solids.Alternatively, the PAE resin can have a charge density from about 2.25to about 3.5 mEq/g of solids; alternatively, from about 2.3 to about3.35 mEq/g of solids; alternatively, from about 2.4 to about 3.2 mEq/gof solids; or alternatively, from about 2.5 to about 3.0 mEq/g ofsolids. The charge density of the polyamidoamine-epihalohydrin resin canbe measured via streaming electrode potential using a Mutek PCDtitrator.

The polyamidoamine-epihalohydrin resin can also have a ratio ofazetidinium ions to amide residues in the PAE resin, which can beabbreviated by “Azet ratio,” from about 0.4 to about 1.3. The Azet ratioalso can be from about 0.5 to about 1.15; alternatively, from about 0.6to about 1.0; or alternatively, from about 0.7 to about 0.9. In afurther aspect, the ratio of azetidinium ions to secondary aminemoieties in the resin can be from about 0.4 to about 1.0. The Azet ratiocan be measured by quantitative ¹³C NMR by comparing the methylenecarbons of the azetidinium versus the methylenes of the acid residue inthe backbone.

In another aspect, the polyamidoamine-epihalohydrin resin can have aweight average molecular weight (Mw) from about 0.02×10⁶ to about3.0×10⁶. Alternatively, the resins that can have a Mw molecular weightfrom about 0.05×10⁶ to about 2.5×10⁶; alternatively, from about 0.1×10⁶to about 2.0×10⁶; alternatively, from about 0.5×10⁶ to about 1.5×10⁶; oralternatively, from about 1×10⁶ to about 1.0×10⁶. In furtherembodiments, the resin that can have a Mw molecular weight from about0.05×10⁶ to about 1.7×10⁶. The Mw molecular weight also can be fromabout 0.6×10⁶ to about 1.6×10⁶; alternatively, from about 0.7×10⁶ toabout 1.5×10⁶; alternatively, from about 0.8×10⁶ to about 1.3×10⁶; oralternatively, from about 0.9×10⁶ to about 1.1×10⁶.

In a further aspect the polyamidoamine-epihalohydrin resin can have anazetidinium equivalent weight, defined as the degree of polymerizationmultiplied times the Azet ratio, or (degree of polymerization)×(Azet),of from about 1,600 to about 3,800. Alternatively, the azetidiniumequivalent weight can be from about 1,800 to about 3,500, oralternatively, from about 2,000 to about 2,900.

One or more urea-formaldehyde (UF) resins can be used as resins for theresin system. A variety of techniques are known in the art for makingurea-formaldehyde (UF) resins can be employed. The urea-formaldehyderesin can be prepared from urea and formaldehyde monomers and/or from UFprecondensates in manners known to those of skill in the art. Forexample, any of the wide variety of procedures used for reacting ureaand formaldehyde monomers to form a UF resin can be used, such as stagedmonomer addition, staged catalyst addition, pH control, aminemodification and the like. The urea and formaldehyde monomers can bereacted in an aqueous solution under alkaline conditions using knowntechniques and equipment.

Formaldehyde for making a suitable UF resin is available in many forms.Formaldehyde suitable for making a PF resin can be available in manyforms. The formaldehyde can be supplied as an aqueous solution known inthe art as “formalin.” Formalin can contain from about 37% to about 50%by weight formaldehyde. Other forms of formaldehyde such asparaformaldehyde also can be used. Other aldehydes can be used in lieuof or in combination with formaldehyde. For example, suitable aldehydesthat can be used in lieu of or in combination with formaldehyde caninclude, but are not limited to, aliphatic aldehydes such asacetaldehyde and propionaldehyde, aromatic aldehydes such asbenzylaldehyde and furfural, glyoxal, crotonaldehyde, or any combinationthereof.

Other aldehyde monomers can be used in lieu of or in combination withformaldehyde for making resins. The aldehyde monomers can include anysuitable aldehyde or combination of aldehydes. The aldehyde monomers caninclude a variety of substituted and unsubstituted aldehyde compounds.Illustrative aldehyde compounds can include the so-called maskedaldehydes or aldehyde equivalents, such as acetals or hemiacetals.Specific examples of suitable aldehyde compounds can include, but arenot limited to, formaldehyde, acetaldehyde, propionaldehyde,butyraldehyde, furfuraldehyde, benzaldehyde, or any combination thereof.As used herein, the term “formaldehyde” can refer to formaldehyde,formaldehyde derivatives, other aldehydes, or combinations thereof.Preferably, the aldehyde monomer can be formaldehyde.

As discussed above, urea is available in many forms that can be used tomake a resin. Solid urea, such as prill, and urea solutions, such asaqueous solutions, can be used. Any form of urea or urea in combinationwith formaldehyde can be uses. Both urea prill and combinedurea-formaldehyde products can be preferred, such as Urea-FormaldehydeConcentrate or UFC 85. These types of products can be as discussed anddescribed in, for example, U.S. Pat. Nos. 5,362,842 and 5,389,716.

The urea-formaldehyde resin can be made using a molar excess offormaldehyde. When synthesized, such resins contain a low level ofresidual “free” urea and a much larger amount of residual “free,” i.e.,unreacted, formaldehyde. Prior to any formaldehyde scavenging, theurea-formaldehyde resin can be characterized by a free formaldehydecontent from about 0.2 wt % to about 18 wt % of the aqueousurea-formaldehyde resin. For example, the urea-formaldehyde resin canhave a concentration of free formaldehyde from a low of about 0.1 wt %,about 0.5 wt %, about 1 wt %, or about 2 wt % to a high of about 6 wt %,about 12 wt %, or about 18 wt %, based on the total weight of theurea-formaldehyde resin.

The urea-formaldehyde resin can have a molar ratio of formaldehyde tourea (F:U) from a low of about 0.3:1, about 0.9:1, or about 1.5:1 to ahigh of about 3:1, about 4:1, about 5:1, or about 6:1. For example, theurea-formaldehyde resin can have a molar ratio of formaldehyde to ureafrom about 0.5:1 to about 0.1.2:1, about 1.3:1 to about 2:1, about 2:1to about 3:1, about 1.1:1 to about 3:1, about 4:1 to about 5:1, or about5:1 to about 6:1. In other example, the urea-formaldehyde resin can havea molar ratio of formaldehyde to urea from about 0.7:1 to about 2.7:1,about 0.9:1 to about 1.3:1, about 1:1 to about 2.4:1, about 1.1:1 toabout 2.6:1, or about 1.3:1 to about 2:1. In another example, theurea-formaldehyde resin can have a molar ratio of formaldehyde to ureafrom about 0.25:2.5 to about 1.5:2.5.

The urea-formaldehyde resin can have a weight average molecular weightfrom a low of about 200, about 300, or about 400 to a high of about1,000, about 2,000, about 14,000, about 25,000, about 50,000, about100,000 or about 500,000. In another example, the urea-formaldehyderesin can have a weight average molecular weight from about 250 to about450, about 450 to about 550, about 550 to about 950, about 950 to about1,500, about 1,500 to about 2,500, or about 2,500 to about 6,000. Inanother example, urea-formaldehyde resin can have a weight averagemolecular weight of about 175 to about 800, about 700 to about 3,330,about 1,100 to about 4,200, about 230 to about 550, about 425 to about875, or about 475 to about 775. In other example, urea-formaldehyderesin can have a weight average molecular weight of about 10,000 toabout 100,000, about 12,000 to about 250,000, or about 14,000 to about500,000.

The reaction can be conducted in an aqueous solution. The reaction canbe conducted so that the resulting urea-formaldehyde resin has a solidscontent of at least about 20 wt %, at least about 30 wt %, at leastabout 35 wt %, or at least about 45 wt %, based on the weight of the UFresin solution. The solids content can range from a low of about 20 wt%, about 30 wt %, about 40 wt %, about 45 wt %, or about 50 wt % to ahigh of about 55 wt %, about 60 wt %, about 65 wt %, about 70 wt %,about 75 wt %, or about 80 wt %, based on the weight of the UF resinsolution. For example, UF resin solutions can have a non-volatilematerial or solids content from about 40 wt % and about 48 wt %, about40 wt % and about 44 wt %, about 45 wt % and about 65 wt %, or about 50wt % and about 60 wt %, based on the weight of the UF resin solution.

The viscosity of the urea-formaldehyde resin can widely vary. Forexample, the viscosity of the urea-formaldehyde resin can range from alow of about 1 cP, about 100 cP, about 250 cP, about 500 cP, or about700 cP to a high of about 1,000 cP, about 1,250 cP, about 1,500 cP,about 2,000 cP, or about 2,200 cP at a temperature of about 25° C. Inanother example, the urea-formaldehyde resin can have a viscosity fromabout 1 cP to about 125 cP, about 125 cP to about 275 cP, about 275 cPto about 525 cP, about 525 cP to about 725 cP, about 725 cP to about1,100 cP, about 1,100 cP to about 1,600 cP, about 1,600 cP to about1,900 cP, or about 1,900 cP to about 2,200 cP at a temperature of about25° C. In another example, the urea-formaldehyde resin can have aviscosity from about 1 cP to about 45 cP, about 45 cP to about 125,about 125 cP to about 550 cP, about 550 cP to about 825 cP, about 825 cPto about 1,100 cP, about 1,100 cP to about 1,600 cP, or about 1,600 cPto about 2,200 cP at a temperature of about 25° C. The viscosity can bedetermined using a Brookfield viscometer. For example, the BrookfieldViscometer can be equipped with a small sample adapter such a 10 mLadapter and the appropriate spindle to maximize torque such as a spindleno. 31.

The urea-formaldehyde resin can have pH from a low of about 1, about 2,about 3, about 4, about 5, about 6, about 7 to a high of about 8, about9, about 10, about 11, about 12, or about 13. In another example,urea-formaldehyde resin can have a pH from about 1 to about 2.5, about2.5 to about 3.5, about 3.5 to about 4.5, about 4.5 to about 5.5, about5.5 to about 6.5, about 6.5 to about 7.5, about 7.5 to about 8.5, about8.5 to about 9.5, about 9.5 to about 10.5, about 10.5 to about 11.5,about 11.5 to about 12.5, or about 12.5 to about 13.

The UF resin can also include additives such as ammonia, alkanolamines,or polyamines, such as an alkyl primary diamine, e.g., ethylenediamine(EDA). Other additives, such as melamine, ethylene ureas, and primary,secondary and tertiary amines, for example, dicyanodiamide, can also beincorporated into UF resins. Concentrations of these additives in thereaction mixture often will vary from about 0.05 to about 20.0% byweight of the UF resin solids. These types of additives can promotehydrolysis resistance, polymer flexibility and lower formaldehydeemissions in the cured resin. Further urea additions for purposes ofscavenging formaldehyde or as a diluent also can be used.Urea-formaldehyde resins can also have a water dilutability of about 1:1to about 100:1, preferably about 5:1 and above.

The additives can be other monomers and/or polymers such as styreneacrylic acid or styrene acrylate, an adduct of styrene, maleicanhydride, and an acrylic acid or acrylate, or a mixture of a styreneacrylic acid or styrene-acrylate copolymer and a styrene-maleicanhydride copolymer. The additive can be added to the UF resin or can beformed in situ by mixing the styrene-maleic anhydride and an acrylatemonomer with the UF resin.

The additive can be prepared by combining styrene, maleic anhydride, andan acrylate or acrylic acid in amounts to form a terpolymer. The amountof styrene can be about 50% to about 85%, preferably about 70%. Theamount of maleic anhydride can be about 15% to about 50%, preferablyabout 25%. The amount of an acrylate or acrylic acid can be about 1 toabout 20%, preferably about 5%.

The constituents of the terpolymer can be dissolved in a suitablesolution such as an aqueous solution of sodium hydroxide, ammoniumhydroxide, potassium hydroxide, or any combination thereof. Preferablyabout 1-5% of the terpolymer constituents can be dissolved in theaqueous solution. The solution can be heated from about 70° C. to about90° C., and held until the terpolymer is in solution. The solution canthen be added to a urea-formaldehyde resin.

Alternatively the acrylic acid or acrylate can be combined with styrenemaleic anhydride in situ with the urea-formaldehyde resin. The resultcan be a styrene maleic anhydride methylmethacrylate terpolymer. Anysuitable acrylic acid or acrylate can be used such as methylmethacrylate, butyl acrylate, or methacrylate. Preferably, the acrylateis methyl methacrylate (MMA). Styrene-maleic anhydride (SMA) copolymerscan be used. Suitable SMA copolymers can be as discussed and describedin U.S. Pat. No. 5,914,365.

The additive can make up about 0.1 wt % to about 10 wt %, preferablyabout 0.5 wt % to about 5 wt % of the undiluted resin solids. The totalconcentration of non-volatile materials in the aqueous resin composition(predominantly UF resin and additive solids) can vary widely. The totalsolids concentration can be about 5 wt % to about 40 wt %, based on thetotal weight of the resin composition. Preferably the total solids canbe from about 20 wt % to about 35 wt %, more preferably from about 20 wt% to about 30 wt %.

Many urea-formaldehyde resins that can be used are commerciallyavailable. One particularly useful class of UF resins for use inpreparing resin systems can include those discussed and described inU.S. Pat. No. 5,362,842. Urea-formaldehyde resins such as the types soldby Georgia Pacific Chemicals LLC (e.g. GP® 2928 and GP® 2980) can beused.

Resin systems and methods for making and using same are provided. In oneembodiment, the resin system can include a first resin, a second resinand pulp fibers. The first resin can be or include one or morepolyamidoamine-epichlorohydrin (PAE) resins or one or moreurea-formaldehyde (UF) resins. The second resin can be or include one ormore polyamidoamine-epichlorohydrin (PAE) resins or one or moreurea-formaldehyde (UF) resins. The first resin can be present in anamount of about 1 wt % to about 99 wt %, based on the total weight theresin system. The second resin can be present in an amount of about 1 wt% to about 99 wt %, based on the total weight the resin system. Thefirst resin or the second resin can be added sequentially orsimultaneously to the pulp fibers, where the period for sequentialaddition between the resins is from about 1 second to about 1 hour. Suchresin systems can be used to enhance the strength of paper, particularlythe wet strength of paper.

In some embodiments, a process of preparing a paper product can includecontacting a plurality of pulp fibers with a resin system. The resinsystem can include a first resin and second resin. The first resin canbe a polyamidoamine-epichlorohydrin (PAE) resin or a urea-formaldehyde(UF) resin. The second resin can be polyamidoamine-epichlorohydrin (PAE)resin or a urea-formaldehyde (UF) resin. The first resin can be presentin an amount of about 1 wt % to about 99 wt %, based on the total weightthe resin system. The second resin can be present in an amount of about1 wt % to about 99 wt %, based on the total weight the resin system. Theprocess can also include adding the first resin or the second resinsequentially or simultaneously to the pulp fibers to produce a paperproduct, where the period for sequential addition between the resins isfrom about 1 second to about 1 hour.

A catalyst or cure accelerator can be added to the resin system in orderto aid in the curing process. Suitable catalysts can include, but arenot limited to, inorganic acids, organic acids (and anhydrides thereof),or any combination thereof. Illustrative inorganic acids can include,but are not limited to, sulfuric acid, hydrochloric acid, phosphoricacid, boric acid, or any combination thereof. Illustrative organic acidsand anhydrides can include, but are not limited to, acetic acid,tartaric acid, benzoic acid, propionic acid, adipic acid, oxalic acid,fumaric acid, hexachloric phthalic anhydride, maleic anhydride, or anycombination thereof. Other catalysts which can be employed can includecompounds that can liberate an acid when heated. Such catalysts caninclude the amine salts of organic and inorganic acids, such as ethylenesulfite, the hydrochloric acid salt of 2-amino-2-methyl propanol, thehydrochloric acid salt of mono-, di-, or triethanol amine, thehydrochloric acid salt of 2-dimethylamino-2-methyl propanol, the aminesalts of para-toluene sulfonic acid, the chloroacetic acid salt ofpyridine, the triammonium acid pyrophosphate salt of aminomethylpropanol, and the phosphoric acid salt of 2-dimethylamino-2-methylpropanol. Other catalysts include the inorganic salts of inorganicacids, such as ammonium chloride, magnesium chloride, zinc chloride, orany combination thereof. The catalyst system can also include mixturesof the aforementioned catalysts.

Catalysts can be added in an amount from about 0.1 wt % to 10 wt %,preferably about 0.1 wt % to 1.5 wt % and, most preferably, about 0.2 wt%, based on the resin solids.

As used herein, the terms “curing,” “cured,” and similar terms areintended to embrace the structural and/or morphological change thatoccurs in a the resin system, such as by covalent chemical reaction(crosslinking), ionic interaction or clustering, improved adhesion tothe substrate, phase transformation or inversion, and/or hydrogenbonding when the resin system can be at least partially cured to causethe properties of a flexible, pulp fibers, to which an effective amountof the resin system has been applied.

In other embodiment, a process of treating paper to impart wet strengthcan include contacting a plurality of paper with a resin system. Theresin system can include a first resin and a second resin. The firstresin can be a polyamidoamine-epichlorohydrin (PAE) resin or aurea-formaldehyde (UF) resin. The second resin can be apolyamidoamine-epichlorohydrin (PAE) resin or a urea-formaldehyde (UF)resin. The first resin can be present in an amount of about 1 wt % toabout 99 wt %, based on the total weight the resin system. The secondresin can be present in an amount of about 1 wt % to about 99 wt %,based on the total weight the resin system. The process can also includeadding the first resin or the second resin sequentially orsimultaneously to the pulp fibers, where the period for sequentialaddition between the resins is from about 1 second to about 1 hour. Theprocess can also include at least partially curing the resin system.

If the first resin and the second resin are sequentially added to thepulp fibers the period for sequential addition between the resin can befrom a low of about 1 second, about 5 seconds, about 10 seconds, about20 seconds, about 30 seconds, about 45 seconds, about 1 minute, about 2minutes, about 3 minutes, about 4 minutes, about 5 minutes or about 10minutes to a high of about 20 minutes, about 30 minutes, about 40minutes, about 50 minutes, or about 60 minutes. For example, the secondresin can be added to the mixture of the first resin and the pulp fibersabout 1 second to about 1 hour, about 1 minute to about 5 minutes, about3 minutes to about 10 minutes, about 5 minutes to about 20 minutes,about 15 minutes to about 30 minutes, about 25 minutes to about 45minutes, or about 30 minutes to about 60 minutes after the first resinwas added to the pulp fibers. In another example, the second resin canbe added to the mixture of the first resin and the pulp fibers at least1 second, at least 5 seconds, at least 10 seconds, at least 20 seconds,at least 30 seconds, at least 45 seconds, at least 1 minute, at least 2minutes, at least 3 minutes, at least 4 minutes, at least 5 minutes, atleast 10 minutes, at least 20 minutes, at least 30 minutes, at least 40minutes, at least 50 minutes and up to about 1 hour, about 1.5 hours,about 2 hours, or about 3 hours after the first resin was added to thepulp fibers.

In at least one embodiment, the first resin can be or include apolyamidoamine-epihalohydrin resin and the second resin can be orinclude a urea-formaldehyde resin and the first and second resins can beadded to the plurality of pulp fibers sequentially with respect to oneanother. For example, the first resin that can include thepolyamidoamine-epihalohydrin resin can be added to the pulp fibers toform a first or intermediate mixture. The second resin that can includethe urea-formaldehyde resin can be added to the first or intermediatemixture to produce a paper product. The first and second resins can beat least partially cured. In at least one other embodiment, the firstresin can be or include a urea-formaldehyde resin and the second resincan be or include a polyamidoamine-epihalohydrin resin and the first andsecond resins can be added to the plurality of pulp fibers sequentiallywith respect to one another. For example, the first resin that caninclude the urea-formaldehyde resin can be added to the pulp fibers toform a first or intermediate mixture. The second resin that can includethe polyamidoamine-epihalohydrin resin can be added to the first orintermediate mixture to produce a paper product. The first and secondresins can be at least partially cured.

In another embodiment, a paper product can include a plurality of pulpfibers and an at least partially cured resin system, where the resinsystem, prior to curing, includes a first resin and second resin. Thefirst resin can be a polyamidoamine-epichlorohydrin (PAE) resin or aurea-formaldehyde (UF) resin. The second resin can bepolyamidoamine-epichlorohydrin (PAE) resin or a urea-formaldehyde (UF)resin. The first resin can be present in an amount of about 1 wt % toabout 99 wt %, based on the total weight the resin system. The secondresin can be present in an amount of about 1 wt % to about 99 wt %,based on the total weight the resin system. The first resin or thesecond resin is added sequentially or simultaneously to the pulp fibers,where the period for sequential addition between the resins is fromabout 1 second to about 1 hour.

The resin systems can be used as adhesives for bonding pulp fibers tomake paper products. Illustrative paper products produced using theresin systems discussed and described herein can include, but are notlimited to, paperboard, tissue, towel, liquid packaging, and the like.In one or more embodiments, the PAE resin can scavenge at least some ofthe free formaldehyde. In one or more embodiments, a blend or mixture ofthe PAE resin and a UF resin can reduce the formaldehyde emission bydilution and/or chemical reaction.

EXAMPLES

In order to provide a better understanding of the foregoing discussion,the following non-limiting examples are offered. Although the examplescan be directed to specific embodiments, they are not to be viewed aslimiting the invention in any specific respect. Unless otherwisespecified, reagents were obtained from commercial sources. The followinganalytical methods were used to characterize the resins.

Example 1 Preparation of Polyamidoamine-Epihalohydrin (PAE) Resin 1

Step 1: To a reaction vessel equipped with an agitator and a refluxcondenser was added 218 g of diethylenetriamine. To this was addedslowly over 45 minutes 318 grams of solid adipic acid. As the adipicacid was added the reaction temperature climbed steadily from ambient to140 C, at which point the temperature remained constant. Upon completionof the adipic acid addition, the reaction mixture was then heated to155° C., at which point reflux began. The reflux condenser wasreconfigured for distillation, and water was distilled from the reactorinto a collection vessel. During the distillation, the reactiontemperature was slowly ramped up to a maximum of 165° C. Distillationwas continued at 165° C. until a sample of the reaction mixture, removedfrom the reactor and diluted to 45% solids, reached a Gardner-Holdtviscosity of L. The distillation condenser was reconfigured for refluxand 350 grams of water was added slowly through the reflux condenser, tocarefully reduce the reaction temperature to approximately 95° C. whilediluting the reaction mixture. Additional water was then added to adjustthe reaction mixture to 45% total solids. The resulting polyamidoaminesolution had a Brookfield viscosity of 300 cP at 45% solids.

Step 2: To 359 gram of the above polyamidoamine solution was added about25 gram of water. Then, 92 gram of epichlorohydrin was gradually addedover 75 minutes under vigorous agitation. The mixture temperature wascontrolled below 25° C. while epichlorohydrin was being added. As theaddition of epichlorohydrin was complete, the mixture then was heated to30° C. and was maintained at the same temperature for 30 minutes. Then387 gram of water to the mixture was added and heated to about 60° C.When the Gardner-Holdt viscosity of the mixture increased to B, thereaction mixture was cooled to 55° C. in order to slow down the reactionrate. The reaction continuously advanced to the Gardner-Holdt viscosityof EF, then was cooled to 50° C. The reaction mixture was maintained at50° C. until it had obtained a viscosity of KL. To the resultingsolution was added about 29.3 gram of an acid mixture containing formicacid and sulfuric acid in a blend ratio of 1.19 to 1 and having an acidconcentration of 52% by weight. As above, the dilution water of 125 gramwas added to achieve the target RI of 1.3826. The final aqueouspolyamidoamine-epichlorohydrin resin resulting solution was obtained byadjusting resin pH to 2.85 using the blend of sulfuric and formic acids.The final resin has a solids concentration of 25.04 wt. %, cationiccharge of 2.09 meq/gram, a pH of 3.0 and a viscosity of 172 cP at 25° C.

Example 2 Preparation of Polyamidoamine-Epihalohydrin (PAE) Resin 2

Step 1: A glass reactor with a 5-neck top was equipped with a stainlesssteel stirring shaft, a reflux condenser, temperature probe, and a hotoil bath was provided. To the reactor was added 500.5 grams of DETA(diethylenetriamine). The stirrer was turned on and 730 grams of adipicacid was added slowly to the reactor over 45 minutes with stirring. Thereaction temperature increased from 25° C. to 145° C. during adipic acidaddition. After the adipic acid addition was complete, the reactor wasimmersed in a hot oil bath heated to 160° C. At 150° C. the reactionmixture began to reflux. The reflux condenser was reconfigured fordistillation, and distillate was collected in a separate receiver. Thereaction mixture was sampled at 30 minute intervals. Each sample wasdiluted to 45% solids with water, and the viscosity was measured withBrookfield viscometer. When the sample reached 290 cP the distillationcondenser was reconfigured to reflux. Water was added slowly to thereaction mixture through the reflux condenser to dilute and cool thereaction. Water was added to obtain a final solids of 45%. The viscositywas 290 cP.

Step 2: A glass reactor with 5-neck top was equipped with a glassstirring shaft and Teflon paddle, an equal pressure addition funnel,temperature and pH probe, stainless steel cooling coils, sample valve,and heating mantle. To the reactor was added 1,000 grams ofPolyamidoamine Prepolymer prepared in step 1. The stirrer was startedand the prepolymer was heated to 40° C. N,N-Methylene-bis-acrylamide,15.16 grams (Pfaltz & Bauer, Inc), was added slowly while the reactionmixture was heated to 60° C. The reaction mixture then was held at 60°C. for about 2 hours, and the viscosity advanced to 4,630 cP(Brookfield-SSA), at which point the viscosity advancement stopped. Thereaction was cooled to 25° C. The intermediate (partially cross-linked)prepolymer was isolated and stored.

Step 3: To the reactor configured as described in Step 2 was added366.04 grams of intermediate (partially cross-linked) prepolymer fromStep 2 above. The reaction temperature was adjusted to 25° C. and 120.13grams of water was added. The viscosity of the reaction mixture was 837cP. To the intermediate partially cross-linked prepolymer was added77.89 grams of epichlorohydrin at 25° C. over 90 minutes. 428.19 Gramsof water was added to the reaction mixture. The reaction was held at 25°C. for 18 hours while sampling periodically for ¹³C NMR analysis. Duringthis time the viscosity of the reaction increased from 18 cP to 319 cP(Brookfield-SSA). This reaction was treated with concentrated sulfuricacid to adjust the pH to 2.94. The reaction mixture was adjusted to25.0% solids, and the viscosity was 335 cP.

Example 3 Preparation of Polyamidoamine-Epihalohydrin (PAE) Resin 3

Step 1: A glass reactor with a 5-neck top was equipped with a stainlesssteel stirring shaft, a reflux condenser, temperature probe, and a hotoil bath was provided. To the reactor was added 1,574.5 grams DBE-5(glutaric acid dimethyl ester, or dibasic ester). The stirrer was turnedon and 1,038.9 grams of DETA was added to the reactor with stirring. Thereactor was immersed in a hot oil bath heated to 100° C. At 90° C. thereaction mixture began to reflux. The reflux condenser was reconfiguredfor distillation and distillate was collected in a separate receiver.The reaction mixture was sampled at 30 minute intervals. Each sample wasdiluted to 45% solids with water, and the viscosity was measured withBrookfield viscometer. When the sample reached 220 cP the distillationcondenser was reconfigured to reflux. Water was added slowly to thereaction mixture through the reflux condenser to dilute and cool thereaction. Water was added to obtain a final solids of 45%. The viscositywas 220 cP.

Step 2: A glass reactor with 5-neck top was equipped with a glassstirring shaft and Teflon paddle, an equal pressure addition funnel,temperature and pH probe, stainless steel cooling coils, sample valve,and heating mantle. To the reactor was added 445.64 grams ofPolyamidoamine Prepolymer from step 1. Water, 5.25 grams was added andthe stirrer was started. The reaction mixture was heated to 35° C. and2.028 grams of N, N-methylene-bis-acrylamide (Pfaltz & Bauer, Inc.) wasadded. The reaction mixture was heated to 60° C. and held at thattemperature for 4 hours. The viscosity of the reaction mixture advancedto 384 cP (Brookfield-SSA). The intermediate (partially cross-linked)prepolymer mixture was utilized in-situ in the following Step 3.

Step 3: The reaction temperature of the intermediate prepolymer mixturefrom Step 2 was adjusted to 25° C., and 88.46 grams of water was added.The reaction temperature was then adjusted to 21° C. and 121.21 grams ofepichlorohydrin was added over 75 minutes. This reaction mixture wasallowed to warm to 25° C. over 45 minutes and 446.27 grams of water wasadded. This reaction mixture was heated to 45° C., and after 2 hours washeated to 55° C. After about 4 hours, a mixture of formic acid andsulfuric acid was added to adjust the pH to 2.87. (Generally, the pH canbe adjusted using any organic acid, mineral acid, or combinationthereof, for example, acetic acid, formic acid, hydrochloric acid,phosphoric acid, sulfuric acid, or any combination thereof.) Thereaction mixture then was cooled to 25° C., and water was added toadjust the solids to 25.0%. The viscosity of the resultant wet strengthresin was 187 cP.

Example 4 Preparation of Polyamidoamine-Epihalohydrin (PAE) Resin 4

Step 1: A glass reactor with a 5-neck top was equipped with a stainlesssteel stirring shaft, a reflux condenser, temperature probe, and a hotoil bath was provided. To the reactor was added 1,574.5 grams DBE-5(glutaric acid dimethyl ester, or dibasic ester). The stirrer was turnedon and 1,038.9 grams of DETA was added to the reactor with stirring. Thereactor was immersed in a hot oil bath heated to 100° C. At 90° C. thereaction mixture began to reflux. The reflux condenser was reconfiguredfor distillation and distillate was collected in a separate receiver.The reaction mixture was sampled at 30 minute intervals. Each sample wasdiluted to 45% solids with water, and the viscosity was measured withBrookfield viscometer. When the sample reached 220 cP the distillationcondenser was reconfigured to reflux. Water was added slowly to thereaction mixture through the reflux condenser to dilute and cool thereaction. Water was added to obtain a final solids of 45%. The viscositywas 220 cP.

Step 2: A glass reactor with 5-neck top was equipped with a glassstirring shaft and Teflon paddle, an equal pressure addition funnel,temperature and pH probe, stainless steel cooling coils, sample valve,and heating mantle. To the reactor was added 449.10 grams ofPolyamidoamine Prepolymer from Step 1. The stirrer was started, thereaction mixture was heated to 30° C., and 6.92 grams of poly(propyleneglycol) diglycidyl ether (Polystar) was added over 1 hour. The reactionmixture held at 30° C. for 1 hour and was then heated to 60° C., atwhich point the viscosity was 416 cP. The reaction mixture was heated at60° C. for about 4 hours, and the viscosity advanced to 542 cP(Brookfield-SSA). The intermediate cross-linked prepolymer was utilizedin-situ in Step 3 that follows.

Step 3: The reaction temperature of the intermediate prepolymer mixturefrom Step 2 was adjusted to 25° C., and 80.10 grams of water was added.To the reactor was added 118.79 grams of epichlorohydrin over 75minutes. The reaction was allowed to warm to 30° C. over 45 minutes, and431.35 grams of water was added. The reaction was warmed to 45° C. over45 minutes and after 2 hours was heated to 50° C. After about 3.5 hoursthe viscosity of the reaction was about 320 cP (Gardner-Holdt bubbletube), and then a mixture of formic acid and sulfuric acid was added toadjust the pH to 3.00. The reaction mixture was cooled to 25° C. andwater was added to adjust the solids to 25.0%. The viscosity of theresultant wet strength resin was 219 cP.

Example 5 Preparation of Urea-Formaldehyde (UF) Resin

A 4 liter glass reactor with a 5-neck top was equipped with a glassstirring shaft and collar, a reflux condenser, a temperature probe, a pHprobe, stainless steel cooling coils, a vacuum sample tube, and aheating mantle. To the reactor was added 852.4 grams of 50%formaldehyde. The stirrer was turned on and 441.2 grams of water wasadded. The pH of the mixture was adjusted with about 0.33 grams of 50%sodium hydroxide to pH 8.5. To the reaction mixture 61.3 grams ofdiethylenetriamine was added over a 20 minute period. The resultingexothermic reaction was controlled to 55° C. with cooling coils. About91 Grams of water was then added to the reaction mixture, and the pH was9.3. With continued cooling 353.4 grams of urea-prill was added. Thereaction temperature was controlled to 62° C. with cooling. The pH ofthe reaction mixture was 8.5. The reaction was then heated to 80° C. andheld at that temperature for 20 minutes, while also keeping the pH atabout 8.3 with the addition of sodium hydroxide as needed. After the 20minute hold period the reaction was treated with 55.7 grams of 18%hydrochloric acid. The reaction was warmed to 83° C. and another 55.7grams of 18% hydrochloric acid was added. The ensuing exothermicreaction brought the temperature up to about 87° C., and the reaction pHwas adjusted to 3.8 with a small amount of 18% hydrochloric acid. Theviscosity of the reaction mixture was checked every 10 minutes usingGardner-Holdt bubble tubes. The viscosity of the reaction increased overabout 1 hour to a Gardner-Holdt E. Water (281.3 grams) was added to thereaction, and the temperature was adjusted to 71° C. with cooling. Overa 1 hour period the reaction pH was allowed to increase to about pH 4.0while cooling to 65° C. During this 1 hour period the viscosity of thereaction advanced to a Gardner-Holdt G, while adding small amounts ofhydrochloric acid to keep the pH at about 4.0. Once a G Gardner-Holdtwas reached 147 grams of water was added and the reaction temperaturewas controlled to 61° C. The reaction viscosity was monitored every 10minutes, and the viscosity advanced to a Gardner-Holdt EF over about 30minutes. At that point 489 grams of water was added and the reactiontemperature was cooled to 50° C. Over a 5 minute period 12 grams ofsodium hydroxide was added while cooling was continued. To the reactionmixture at that time was added 89.8 grams of urea-prill. The reactiontemperature was then adjusted to 45° C. over a 60 minute period. The pHof the reaction was then adjusted to about 6.5 with a small amount of50% sodium hydroxide, and this was followed by the addition of 143 gramsof water. The completed reaction was then cooled to 25° C. and theconcentration of the reaction was adjusted to 25% by the addition ofwater as needed. The final UF resin sample at 25% solids had a pH of6.5, a Brookfield small sample adapter viscosity of 20 cP. The resin wastested with the sodium sulfite-ice method for free formaldehyde and wasfound to contain 0.2%.

Example 6 Evaluation of Resin System's Properties and Performance

Handsheets were prepared and tested for physical properties: wet tensileand repulpability. The pulp stock used was an unbleached Kraft obtainedfrom a commercial paperboard machine. The stock freeness was in therange of 390 to 410 CSF. The stock pH was 5.2 through the process. Thecomposition resins were added at 10 lb/ton of pulp solids to a 0.37%consistency diluted stock. When only PAE(polyamidoamine-epichlorohydrin) or UF (urea-formaldehyde) resin wasadded at 10 lb/ton the resin was added with a one minute mixing time.When the resins were added sequentially, the PAE was added for a oneminute mixing time followed by addition of the UF resin for anadditional minute. When the resins were added sequentially the combinedtotal was always 10 lb/ton. The treated stock was immediately pouredinto the headbox of the Noble & Wood handsheet machine containing pHpre-adjusted water (pH of 5.2). The target basis weight was 35 lb/3,000ft². Each sheet was passed once @ 20 psi between two blotters throughthe Adirondack wet press followed by five passes through the Adirondackdrum dryer at 240° C. All sets of handsheets were further cured for tenminutes at 105° C. in a forced air oven. The handsheet samples wereequilibrated at a constant humidity (50%) and at a constant temperature(73° F.) for twenty-four hours prior to testing. Wet tensile (testspecimens immersed in distilled water at 23.0±2° C. under the vacuumlevel of 21 inch Hg for saturation) were tested to measure improved wettensile strength performance. Wet tensile measurement method refers toTAPPI Test Method 456 om-10. Repulpability method refers to “VoluntaryStandard” For Repulping and Recycling Corrugated Fiberboard Treated toImprove Its Performance in the Presence of Water and Water Vapor” issuedby the Fibre Box Association (FBA).

Table 1 illustrates that the resin systems prepared according to thisdisclosure show significant improvement in actual wet tensile strengthand repulpability of paper. The PAE resin used in this Example was PAEResin 1 discussed in Example 1 above and the UF resin was the UF resindiscussed in Example 5 above. Table 1 shows actual and theoretical wettensile strength and repulpability data for resin addition at ten lb/tonfor PAE only, ten lb/ton for UF only, and sequential additions of resins(ten lb/ton total) for percent ratios of 70:30, 50:50 and 30:70(PAE:UF).

TABLE 1 Properties of resin system (actual wet tensile strength vs.theoretical wet tensile strength & repulpability) via sequentialaddition of PAE & UF Actual Wet Theoretical PAE UF Tensile Wet TensileConcen- Concen- Strength Strength Repulpability tration tration(Lbf/inch) (Lbf/inch) % Accepts 100 0 3.45 94 70 30 2.89 2.89 98 50 502.91 2.53 99 30 70 2.42 2.16 100 0 100 1.61 100

The data in Table 1 indicates that above 30% UF (or below 70% PAE) asynergy occurs in that the actual wet tensile is higher than thetheoretical value. The repulpability of PAE and UF resin system is at orover 98%. This increase in wet tensile strength was both surprising andunexpected.

Example 7 Evaluation of Resin System's Properties and Performance

Handsheets were prepared and tested for wet tensile. The pulp stock usedwas an unbleached Kraft obtained from a commercial paperboard machine.The stock freeness was in the range of 420 to 440 CSF. The stock pH was5.3 through the process. The composition resins were added at ten lb/tonof pulp solids to a 0.45% consistency diluted stock. When only PAE(polyamidoamine) or UF (urea-formaldehyde) resin was added at ten lb/tonthe resin was added with a one minute mixing time. When the resins wereadded sequentially, the PAE was added for a one minute mixing timefollowed by addition of the UF resin for an additional minute. When theresins were added sequentially the combined total was always ten lb/ton.The treated stock was immediately poured into the headbox of the Noble &Wood handsheet machine containing pH pre-adjusted water (pH of 5.3). Thetarget basis weight was 35 lb/3,000 ft². Each sheet was passed once @ 20psi between two blotters through the Adirondack wet press followed byfive passes through the Adirondack drum dryer at 240° C. All sets ofhandsheets were further cured for ten minutes at 105° C. in a forced airoven. The handsheet samples were equilibrated at a constant humidity(50%) and at a constant temperature (73° F.) for twenty-four hours priorto testing. Wet tensile (test specimens immersed in distilled water at23.0±2° C. under the vacuum level of 21 inch Hg for saturation) weretested to measure improved wet tensile strength performance. Wet tensilemeasurement method refers to TAPPI Test Method 456 om-10. The followingtable and graph provides wet tensile data for waterleaf (no resinaddition) plus resin addition at ten lb/ton for PAE only, ten lb/ton UFonly and sequential additions of resins (ten lb/ton total) for percentratios of 50:50, 35:65, and 20:80 (PAE:UF).

Table 2 illustrates that the resin systems prepared according to thisdisclosure show significant improvement in actual wet tensile strengthpaper. The PAE resin used in this Example was PAE Resin 1 discussed inExample 1 above and the UF resin was the UF resin discussed in Example 5above. Table 2 shows actual and theoretical wet tensile strength forresin addition at ten lb/ton for PAE only, ten lb/ton for UF only andsequential additions of resins (ten lb/ton total) for percent ratios of50:50, 35:65 and 20:8 (PAE:UF).

TABLE 2 Properties of resin system (actual wet tensile strength vs.theoretical wet tensile strength) via sequential addition of PAE & UFActual Wet Theoretical Tensile Wet Tensile PAE UF Strength StrengthConcentration Concentration (Lbf/inch) (Lbf/inch) 100 0 9.31 50 50 7.976.1 35 65 6.27 5.14 20 80 4.48 4.17 0 100 2.89

The data shown in Table 2 indicates that above 50% UF (or below 50% PAE)a synergy occurs in that the actual wet tensile is higher thantheoretical value. This increase in wet tensile strength was bothsurprising and unexpected.

It should be noted that the wet tensile strength values for the tenlb/ton for PAE only and the ten lb/ton for UF only shown in Table 2(Example 7) were quite a bit higher than those shown in Table 1 (Example6), where ten lb/ton PAE only and ten lb/ton UF only were also used tomake the same handsheets. Without wishing to be bound by theory, it isbelieved that a difference in the furnish between Example 6 and 7 is thereason for the difference in wet tensile strength, which should haveproduced the same or very similar values. It is speculated that thefurnish in Example 6 was somehow contaminated that resulted in the lowerwet tensile values.

Example 8 Evaluation of Resin System's Properties and Performance

Handsheets were prepared and tested for wet tensile. The pulp stock usedwas an unbleached Kraft obtained from a commercial paperboard machine.The stock freeness was in the range of 420 to 440 CSF. The stock pH was5.3 through the process. The composition resins were added at 5 lb/tonof pulp solids to a 0.45% consistency diluted stock. When only PAE(polyamidoamine) or UF (urea-formaldehyde) resin was added at 5 lb/ton,the resin was added with a one minute mixing time. When the resins wereadded sequentially, the PAE was added for a one minute mixing timefollowed by addition of the UF resin for an additional minute. When theresins were added sequentially the combined total was always 5 lb/ton.The treated stock was immediately poured into the headbox of the Noble &Wood handsheet machine containing pH pre-adjusted water (pH of 5.3). Thetarget basis weight was 35 lb/3,000 ft². Each sheet was passed once @ 20psi between two blotters through the Adirondack wet press followed byfive passes through the Adirondack drum dryer at 240° C. All sets ofhandsheets were further cured for ten minutes at 105° C. in a forced airoven. The handsheet samples were equilibrated at a constant humidity(50%) and at a constant temperature (73° F.) for twenty-four hours priorto testing. Wet tensile (test specimens immersed in distilled water at23.0±2° C. under the vacuum level of 21 inch Hg for saturation) weretested to measure improved wet tensile strength performance. Wet tensilemeasurement method refers to TAPPI Test Method 456 om-10. The followingtable and graph provides wet tensile data for waterleaf (no resinaddition) plus resin addition at five lb/ton for PAE only, five lb/tonUF only and sequential additions of resins (five lb/ton total) forpercent ratios of 50:50, 35:65, 20:80 (PAE:UF).

Table 3 illustrates that the resin systems prepared according to thisdisclosure show significant improvement in actual wet tensile strengthof paper. The PAE resin used in this Example was PAE Resin 1 discussedin Example 1 above and the UF resin was the UF resin discussed inExample 5 above. Table 3 shows actual and theoretical wet tensilestrength for resin addition at 5 lb/ton for PAE only, 5 lb/ton for UFonly, and sequential additions of resins (five lb/ton total) for percentratios of 50:50, 35:65 and 20:80 (PAE:UF).

TABLE 3 Properties of resin system (actual wet tensile strength vs.theoretical wet tensile strength) via sequential addition of PAE & UFActual Wet Theoretical Tensile Wet Tensile PAE UF Strength StrengthConcentration Concentration (Lbf/inch) (Lbf/inch) 100 0 7.10 50 50 5.784.71 35 65 4.93 3.99 20 80 4.63 3.27 0 100 2.31

The data shown in Table 3 indicates that above 50% UF (or below 50% PAE)a synergy occurs in that the actual wet tensile is higher thantheoretical value. This increase in wet tensile strength was bothsurprising and unexpected.

Embodiments of the present disclosure further relate to any one or moreof the following paragraphs:

1. A resin system for enhancing the wet strength of paper comprising: afirst resin, wherein: the first resin is present in an amount of about 1wt % to about 99 wt %, based on the total weight the resin system; and asecond resin; wherein the second resin is present in an amount of about1 wt % to about 99 wt %, based on the total weight the resin system;wherein the first resin or the second resin are added sequentially orsimultaneously to the pulp fibers; and wherein the period for sequentialaddition between the resins is from about 1 second to about 1 hour.

2. The resin system according to paragraph 1, wherein the first resin isa conventional polyamidoamine-epihalohydrin resin or a non-conventionalpolyamidoamine-epihalohydrin resin.

3. The resin system according to paragraph 2, wherein thenon-conventional polyamidoamine-epihalohydrin resin is prepared by theprocess comprising: a) reacting a polyamine with a symmetriccross-linker to produce a partially cross-linked polyamine; b) adding aepihalohydrin to the partially cross-linked polyamine to produce ahalohydrin-functionalized polymer; and c) cyclizing thehalohydrin-functionalized polymer to form the resin having azetidiniummoieties.

4. The resin system according to paragraph 3, wherein the polyamine hasthe structure

wherein R is alkyl, hydroxyalkyl, amine, amide, aryl, heteroaryl orcycloalkyl and w is an integer from 1 to about 10,000.

5. The resin system according to paragraph 3 or 4, wherein the symmetriccross-linker is selected from a di-acrylate, a bis(acrylamide), adi-epoxide, N,N′-methylene-bis-acrylamide,N,N′-methylene-bis-methacrylamide, poly(ethylene glycol) diglycidylether, poly(propylene glycol) diglycidyl ether, polyethylene glycoldiacrylate, polyazetidinium compounds and any combination thereof.

6. The resin system according to paragraph 3 or 4, wherein the symmetriccross-linker is selected from:

wherein R⁴ is (CH₂)_(t), and wherein t is 1, 2, or 3;

wherein x is from about 1 to about 100;

wherein y is from about 1 to about 100;

wherein x′+y′ is from about 1 to about 100;

wherein z is from about 1 to about 100;

wherein a q/p ratio is from about 10 to about 1000; a copolymer of anacrylate monomer, a methacrylate monomer, an alkene monomer, or a dienemonomer with an azetidinium-functionalized monomer selected from

and a combination thereof, wherein the fraction ofazetidinium-functionalized monomer to the acrylate monomer, themethacrylate monomer, the alkene monomer, or the diene monomer in thecopolymer is from about 0.1% to about 12%; and any combination thereof.

7. The resin system according to any one of paragraphs 3 to 6, whereinthe epihalohydrin is selected from epichlorohydrin, epibromohydrin, andepiiodohydrin.

8. The resin system according to any one of paragraphs 3 to 7, furthercomprising: reacting the polyamine with a mono-functional modifier priorto, during, or after treating with the symmetric cross-linker.

9. The resin system according to paragraph 8, wherein themono-functional modifier is selected from alkyl acrylate, hydroxyalkylacrylate, 2-(2-hydroxyethoxy)ethyl acrylate, acrylamide, alkylacrylamide, N-alkylacrylamide, dialkyl acrylamide,N,N-dialkylacrylamide, acrylonitrile, 2-alkyl oxirane,2-(allyloxyalkyl)oxirane, 2-(allyloxymethyl)oxirane,ω-(acryloyloxy)-alkyltrimethylammonium,ω-(acrylamido)-alkyltrimethylammonium, mono-epoxide,1-isopropyl-3-(methacryloyloxy)-1-methylazetidinium chloride and anycombination thereof.

10. The resin system according to any one of paragraphs 2 to 9, whereinthe polyamidoamine-epihalohydrin resin has solid contents from about 10%to about 50%.

11. The resin system according to any one of paragraphs 2 to 10, whereinthe polyamidoamine-epihalohydrin resin has a molecular weight from about0.02×10⁶ to about 3.0×10⁶.

12. The resin system according to any one of paragraphs 1 to 11, whereinthe second resin is a urea-formaldehyde resin.

13. The resin system according to paragraph 12, wherein theurea-formaldehyde resin has a molar ratio of formaldehyde to urea fromabout 1.5 to about 2.5.

14. The resin system according to paragraph 12 or 13, wherein theurea-formaldehyde resin has solid contents from about 10% to about 50%.

15. The resin system according to any one of paragraphs 12 to 14,wherein the urea-formaldehyde resin has a weight average molecularweight from about 14,000 to about 500,000.

16. The resin system according to any one of paragraphs 1 to 15, whereinthe first resin or the second resin is in aqueous form or in solutionform.

17. The resin system according to any one of paragraphs 1 to 16, whereinthe period for sequential addition between the resins is 1 minute.

18. The resin system according to any one of paragraphs 1 to 17, whereinthe period for sequential addition between the resins is 5 minutes.

19. The resin system according to any one of paragraphs 1 to 18, whereinthe period for sequential addition between the resins is 10 minutes.

20. A process of preparing a paper product, comprising: contacting aplurality of pulp fibers with a resin system comprising: a first resinand a second resin, wherein: the first resin is present in an amount ofabout 1 wt % to about 99 wt %, based on the total weight the resinsystem; the second resin is present in an amount of about 1 wt % toabout 99 wt %, based on the total weight the resin system; and addingthe first resin or the second resin sequentially or simultaneously tothe pulp fibers to produce a paper product; wherein the period forsequential addition between the resins is from about 1 second to about 1hour.

21. The process according to paragraph 20, wherein the first resin is aconventional polyamidoamine-epihalohydrin resin or a non-conventionalpolyamidoamine-epihalohydrin resin using crosslinkers.

22. The process according to paragraph 21, wherein the non-conventionalpolyamidoamine-epihalohydrin resin is prepared by the processcomprising: a) reacting a polyamine with a symmetric cross-linker toproduce a partially cross-linked polyamine; b) adding a epihalohydrin tothe partially cross-linked polyamine to produce ahalohydrin-functionalized polymer; and c) cyclizing thehalohydrin-functionalized polymer to form the resin having azetidiniummoieties.

23. The process according to paragraph 22, wherein the polyamine has thestructure

wherein R is alkyl, hydroxyalkyl, amine, amide, aryl, heteroaryl orcycloalkyl and w is an integer from 1 to about 10,000.

24. The process of claim 22, wherein the symmetric cross-linker isselected from a di-acrylate, a bis(acrylamide), a di-epoxide,N,N′-methylene-bis-acrylamide, N,N′-methylene-bis-methacrylamide,poly(ethylene glycol) diglycidyl ether, poly(propylene glycol)diglycidyl ether, polyethylene glycol diacrylate, polyazetidiniumcompounds and any combination thereof.

25. The process according to paragraph 22, wherein the symmetriccross-linker is selected from:

wherein R⁴ is (CH₂)_(t), and wherein t is 1, 2, or 3;

wherein x is from about 1 to about 100;

wherein y is from about 1 to about 100;

wherein x′+y′ is from about 1 to about 100;

wherein z is from about 1 to about 100;

wherein a q/p ratio is from about 10 to about 1000; a copolymer of anacrylate monomer, a methacrylate monomer, an alkene monomer, or a dienemonomer with an azetidinium-functionalized monomer selected from

and a combination thereof, wherein the fraction ofazetidinium-functionalized monomer to the acrylate monomer, themethacrylate monomer, the alkene monomer, or the diene monomer in thecopolymer is from about 0.1% to about 12%; and any combination thereof.

26. The process according to any one of paragraphs 22 to 25, wherein theepihalohydrin is selected from epichlorohydrin, epibromohydrin, andepiiodohydrin.

27. The process according to any one of paragraphs 22 to 26, furthercomprising: reacting the polyamine with a mono-functional modifier priorto, during, or after treating with the symmetric cross-linker.

28. The process according to paragraph 27, wherein the mono-functionalmodifier is selected from alkyl acrylate, hydroxyalkyl acrylate,2-(2-hydroxyethoxy)ethyl acrylate, acrylamide, alkyl acrylamide,N-alkylacrylamide, dialkyl acrylamide, N,N-dialkylacrylamide,acrylonitrile, 2-alkyl oxirane, 2-(allyloxyalkyl)oxirane,2-(allyloxymethyl)oxirane, ω-(acryloyloxy)-alkyltrimethylammonium,ω-(acrylamido)-alkyltrimethylammonium, mono-epoxide,1-isopropyl-3-(methacryloyloxy)-1-methylazetidinium chloride and anycombination thereof.

29. The process according to any one of paragraphs 21 to 28, wherein thepolyamidoamine-epihalohydrin resin has solid contents from about 10% toabout 50%.

30. The process according to any one of paragraphs 21 to 29, wherein thepolyamidoamine-epihalohydrin resin has a molecular weight from about0.02×10⁶ to about 3.0×10⁶.

31. The process according to any one of paragraphs 20 to 30, wherein thesecond resin is a urea-formaldehyde resin.

32. The process according to paragraph 31, wherein the urea-formaldehyderesin has a molar ratio of formaldehyde to urea from about 1.5 to about2.5.

33. The process according to paragraph 31 or 32, wherein theurea-formaldehyde resin has solid contents from about 10% to about 50%.

34. The process according to any one of paragraphs 31 to 33, wherein theurea-formaldehyde resin has a weight average molecular weight from about14,000 to about 500,000.

35. The process according to any one of paragraphs 20 to 34, wherein thefirst resin or the second resin is in aqueous form or in solution form.

36. The process according to any one of paragraphs 20 to 35, wherein theperiod for sequential addition between the resins is 1 minute.

37. The process according to any one of paragraphs 20 to 36, wherein theperiod for sequential addition between the resins is 5 minutes.

38. The process according to any one of paragraphs 20 to 37, wherein theperiod for sequential addition between the resins is 10 minutes.

39. A paper strengthened with the resin system according to any one ofparagraphs 1 to 19.

40. A process of treating paper to impart wet strength, comprising:contacting a plurality of paper with a resin system comprising: a firstresin and a second resin, wherein: the first resin is present in anamount of about 1 wt % to about 99 wt %, based on the total weight theresin system; the second resin is present in an amount of about 1 wt %to about 99 wt %, based on the total weight the resin system; adding thefirst resin or the second resin sequentially or simultaneously to thepulp fibers; wherein the period for sequential addition between theresins is from about 1 second to about 1 hour; and at least partiallycuring the resin system.

41. A paper product, comprising: a plurality of pulp fibers and an atleast partially cured resin system, wherein the resin system, prior tocuring, comprises: a first resin, wherein: the first resin is present inan amount of about 1 wt % to about 99 wt %, based on the total weightthe resin system; and a second resin; wherein the second resin ispresent in an amount of about 1 wt % to about 99 wt %, based on thetotal weight the resin system; wherein the first resin or the secondresin is added sequentially or simultaneously to the pulp fibers; andwherein the period for sequential addition between the resins is fromabout 1 second to about 1 hour.

42. A method for making a paper product, comprising: contacting aplurality of pulp fibers with a resin system comprising a firstpolyamidoamine-epihalohydrin resin and a second resin comprising asecond polyamidoamine-epihalohydrin resin, a urea-formaldehyde resin, ora mixture thereof to produce a paper product, wherein the first resinand the second resin are sequentially or simultaneously contacted withthe plurality of pulp fibers, and wherein the period for sequentialaddition between the first resin and the second resin is about 1 secondto about 1 hour.

43. A paper product, comprising: a plurality of pulp fibers and an atleast partially cured resin system, wherein the resin system, prior tocuring, comprises a first polyamidoamine-epihalohydrin resin and asecond resin comprising a second polyamidoamine-epihalohydrin resin, aurea-formaldehyde resin, or a mixture thereof, wherein the first resinand the second resin are sequentially or simultaneously contacted withthe plurality of pulp fibers, and wherein the period for sequentialaddition between the first resin and the second resin is about 1 secondto about 1 hour.

44. A composition comprising: a plurality of pulp fibers; and a resinsystem comprising a first polyamidoamine-epihalohydrin resin and asecond resin comprising a second polyamidoamine-epihalohydrin resin, aurea-formaldehyde resin, or a mixture thereof, wherein the compositionis made by contacting first resin and the second resin sequentially orsimultaneously with the plurality of pulp fibers, and wherein the periodfor sequential addition between the first resin and the second resin isabout 1 second to about 1 hour.

45. The method, product, or composition according to any one ofparagraphs 42 to 44, wherein the plurality of pulp fibers issequentially contacted with the first polyamidoamine-epihalohydrin resinfollowed by the second resin.

46. The method, product, or composition according to any one ofparagraphs 42 to 45, wherein the plurality of pulp fibers issequentially contacted with the first polyamidoamine-epihalohydrin resinfollowed by the second resin, and wherein the period for sequentialaddition between the first resin and the second resin is about 1 minuteto about 15 minutes.

47. The method, product, or composition according to any one ofparagraphs 42 to 46, wherein the first polyamidoamine-epihalohydrinresin has a pH of about 2 to about 4.5, a charge density of about 2mEq/g of solids to about 4 mEq/g of solids, and a ratio of azetidiniummoieties to amide residues of about 0.4 to about 1.3.

48. The method, product, or composition according to any one ofparagraphs 42 to 47, wherein the first polyamidoamine-epihalohydrinresin comprises azetidinium moieties formed by cyclizing ahalohydrin-functionalized polymer, wherein the halohydrin-functionalizedpolymer comprises halohydrin groups, and wherein about 90% or more ofthe halohydrin groups in the halohydrin-functionalized polymer arecyclized to form the azetidinium moieties.

49. The method, product, or composition according to any one ofparagraphs 42 to 48, wherein the second resin comprises theurea-formaldehyde resin.

50. The method, product, or composition according to any one ofparagraphs 42 to 47, wherein the first polyamidoamine-epihalohydrinresin is made by reacting a polyamine with a functionally-symmetriccross-linker to produce a partially cross-linked polyamine; reacting anepihalohydrin with the partially cross-linked polyamine to produce ahalohydrin-functionalized polymer; and cyclizing thehalohydrin-functionalized polymer to produce azetidium moieties.

51. The method, product, or composition according to paragraph 50,wherein the second resin comprises the urea-formaldehyde resin.

52. The method, product, or composition according to any one ofparagraphs 42 to 51, wherein the epihalohydrin comprisesepichlorohydrin, epibromohydrin, epiiodohydrin, or any mixture thereof.

53. The method, product, or composition according to any one ofparagraphs 42 to 52, wherein the first polyamidoamine-epihalohydrinresin has solid content from about 10 wt % to about 50 wt %, a weightaverage molecular weight of about 0.02×10⁶ to about 3.0×10⁶, wherein thesecond resin comprises a urea-formaldehyde resin having a molar ratio offormaldehyde to urea of about 1.5 to about 2.5 and a solids content ofabout 10% to about 50%, and wherein the second resin is sequentiallycontacted with the pulp fibers with respect to the firstpolyamidoamine-epihalohydrin resin.

54. The method, product, or composition according to any one ofparagraphs 42 to 53, wherein the polyamine has the structure

wherein R is alkyl, hydroxyalkyl, amine, amide, aryl, heteroaryl orcycloalkyl and w is an integer from 1 to about 10,000.

55. The method, product, or composition according to any one ofparagraphs 42 to 54, wherein the functionally-symmetric cross-linkercomprises a di-acrylate, a bis(acrylamide), a di-epoxide,N,N′-methylene-bis-acrylamide, N,N′-methylene-bis-methacrylamide,poly(ethylene glycol) diglycidyl ether, poly(propylene glycol)diglycidyl ether, polyethylene glycol diacrylate, polyazetidiniumcompounds, or any combination thereof.

56. The method, product, or composition according to any one ofparagraphs 42 to 55, wherein the functionally-symmetric cross-linkercomprises:

wherein R⁴ is (CH₂)_(t), and wherein t is 1, 2, or 3;

wherein x is from about 1 to about 100;

wherein y is from about 1 to about 100;

wherein x′+y′ is from about 1 to about 100;

wherein z is from about 1 to about 100;

wherein a q/p ratio is from about 10 to about 1000; a copolymer of anacrylate monomer, a methacrylate monomer, an alkene monomer, or a dienemonomer with an azetidinium-functionalized monomer selected from

or any mixture thereof, wherein the fraction ofazetidinium-functionalized monomer to the acrylate monomer, themethacrylate monomer, the alkene monomer, or the diene monomer in thecopolymer is from about 0.1% to about 12%; and any mixture thereof.

57. The method, product, or composition according to any one ofparagraphs 42 to 56, further comprising reacting the polyamine with amono-functional modifier prior to, during, or after reacting with thefunctionally-symmetric cross-linker.

58. The method, product, or composition according to paragraph 57,wherein the mono-functional modifier is selected from alkyl acrylate,hydroxyalkyl acrylate, 2-(2-hydroxyethoxy)ethyl acrylate, acrylamide,alkyl acrylamide, N-alkylacrylamide, dialkyl acrylamide,N,N-dialkylacrylamide, acrylonitrile, 2-alkyl oxirane,2-(allyloxyalkyl)oxirane, 2-(allyloxymethyl)oxirane,ω-(acryloyloxy)-alkyltrimethylammonium,ω-(acrylamido)-alkyltrimethylammonium, mono-epoxide,1-isopropyl-3-(methacryloyloxy)-1-methylazetidinium chloride, and anymixture thereof.

59. The method, product, or composition according to any one ofparagraphs 42 to 58, wherein the first polyamidoamine-epihalohydrinresin has a solids content of about 10% to about 50% and a weightaverage molecular weight of about 0.02×10⁶ to about 3.0×10⁶.

60. The method, product, or composition according to paragraph 59,wherein the second resin comprises the urea-formaldehyde resin, whereinthe urea-formaldehyde resin has a molar ratio of formaldehyde to urea ofabout 1.5 to about 2.5 and a weight average molecular weight of about14,000 to about 500,000.

61. The method, product, or composition according to any one ofparagraphs 42 to 60, further comprising at least partially curing theresin system to produce the paper product.

Certain embodiments and features have been described using a set ofnumerical upper limits and a set of numerical lower limits. It should beappreciated that ranges including the combination of any two values,e.g., the combination of any lower value with any upper value, thecombination of any two lower values, and/or the combination of any twoupper values are contemplated unless otherwise indicated. Certain lowerlimits, upper limits and ranges appear in one or more claims below. Allnumerical values are “about” or “approximately” the indicated value, andtake into account experimental error and variations that would beexpected by a person having ordinary skill in the art.

Various terms have been defined above. To the extent a term used in aclaim is not defined above, it should be given the broadest definitionpersons in the pertinent art have given that term as reflected in atleast one printed publication or issued patent. Furthermore, allpatents, test procedures, and other documents cited in this applicationare fully incorporated by reference to the extent such disclosure is notinconsistent with this application and for all jurisdictions in whichsuch incorporation is permitted.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention can be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A resin system for making paper products,comprising a polyamidoamine-epihalohydrin resin and a urea-formaldehyderesin, wherein the resin system comprises greater than 30 wt % to about80 wt % of the urea-formaldehyde resin, based on a combined solidsweight of the polyamidoamine-epihalohydrin resin and theurea-formaldehyde resin.
 2. The resin system of claim 1, wherein thepolyamidoamine-epihalohydrin resin has a pH of about 2 to about 4.5, acharge density of about 2 mEq/g of solids to about 4 mEq/g of solids,and a ratio of azetidinium moieties to amide residues of about 0.4 toabout 1.3.
 3. The resin system of claim 1, wherein thepolyamidoamine-epihalohydrin resin has a pH of about 2 to about 4.5, acharge density of about 2.5 mEq/g of solids to about 4 mEq/g of solids,and a ratio of azetidinium moieties to amide residues of about 0.6 toabout
 1. 4. The resin system of claim 1, wherein the resin systemcomprises about 40 wt % to about 80 wt % of the urea-formaldehyde resin,based on the combined solids weight of the polyamidoamine-epihalohydrinresin and the urea-formaldehyde resin.
 5. The resin system of claim 1,wherein the polyamidoamine-epihalohydrin resin comprises azetidiniumions and at least one polyamine that is partially crosslinked with abridging moiety, and wherein the bridging moiety is derived from afunctionally-symmetric cross-linker comprising a di-acrylate compound, abis(acrylamide) compound, N,N′-methylene-bis-methacrylamide, adi-epoxide compound, a polyazetidinium compound, a poly(alkylene glycol)diglycidyl ether, or any mixture thereof.
 6. The resin system of claim5, wherein the polyamine has the chemical formula:

wherein R is an alkyl, a hydroxyalkyl, an amine, an amide, an aryl, aheteroaryl, or a cycloalkyl and w is an integer from 1 to about 10,000.7. The resin system of claim 1, wherein the polyamidoamine-epihalohydrinresin comprises azetidinium ions and at least one polyamine that ispartially crosslinked with a bridging moiety, and wherein the bridgingmoiety is derived from a functionally-symmetric cross-linker comprisinga di-acrylate compound, a bis(acrylamide) compound,N,N′-methylene-bis-methacrylamide, a di-epoxide compound, poly(ethyleneglycol) diglycidyl ether, a poly(propylene glycol) diglycidyl ether, orany mixture thereof.
 8. The resin system of claim 1, wherein thepolyamidoamine-epihalohydrin resin comprises azetidinium ions and atleast one polyamine that is partially crosslinked with a bridgingmoiety, wherein the bridging moiety is derived from afunctionally-symmetric cross-linker, and wherein thefunctionally-symmetric cross-linker comprises:

wherein R⁴ is (CH₂)_(t), and wherein t is 1, 2, or 3;

wherein x is from about 1 to about 100;

wherein y is from about 1 to about 100;

wherein x′+y′ is from about 1 to about 100;

wherein z is from about 1 to about 100;

wherein a q/p ratio is from about 10 to about 1,000; a copolymer of anacrylate monomer, a methacrylate monomer, an alkene monomer, or a dienemonomer with an azetidinium-functionalized monomer comprising:

or a mixture thereof; or any mixture thereof.
 9. The resin system ofclaim 1, wherein the urea-formaldehyde resin has a molar ratio offormaldehyde to urea of about 1:1 to about 3:1 and a weight averagemolecular weight of about 1,000 to about 500,000.
 10. The resin systemof claim 1, wherein a synthesis of the polyamidoamine-epihalohydrinresin comprises reacting a polyamine with a functionally-symmetriccross-linker to produce a partially cross-linked polyamine, reacting anepihalohydrin with the partially cross-linked polyamine to produce ahalohydrin-functionalized polymer, and cyclizing thehalohydrin-functionalized polymer, and wherein thefunctionally-symmetric cross-linker comprises a di-acrylate compound, abis(acrylamide) compound, N,N′-methylene-bis-methacrylamide, adi-epoxide compound, a polyazetidinium compound, a poly(alkylene glycol)diglycidyl ether, or any mixture thereof.
 11. The resin system of claim10, wherein the synthesis of the polyamidoamine-epihalohydrin resinfurther comprises reacting the polyamine with a mono-functional modifierprior to, during, or after the polyamine is reacted with thefunctionally-symmetric cross-linker.
 12. The resin system of claim 11,wherein the mono-functional modifier comprises an alkyl acrylate,acrylamide, an alkyl acrylamide, a dialkyl acrylamide, acrylonitrile, a2-alkyl oxirane, a 2-(allyloxyalkyl)oxirane, a hydroxyalkyl acrylate, anω-(acryloyloxy)-alkyltrimethylammonium compound, anω-(acrylamido)-alkyltrimethylammonium compound, or any mixture thereof.13. The resin system of claim 1, wherein the resin system furthercomprises a catalyst, and wherein the catalyst comprises sulfuric acid,hydrochloric acid, phosphoric acid, boric acid, acetic acid, tartaricacid, benzoic acid, propionic acid, adipic acid, oxalic acid, fumaricacid, hexachloric phthalic anhydride, maleic anhydride, ammoniumchloride, magnesium chloride, zinc chloride, or any mixture thereof. 14.A resin system for making paper products, comprising a firstpolyamidoamine-epihalohydrin resin and a second resin comprising asecond polyamidoamine-epihalohydrin resin, a urea-formaldehyde resin, ora mixture thereof, wherein the first polyamidoamine-epihalohydrin resincomprises azetidinium ions and at least one polyamine that is partiallycrosslinked with a bridging moiety, wherein the bridging moiety isderived from a functionally-symmetric cross-linker comprising adi-acrylate compound, a bis(acrylamide) compound,N,N′-methylene-bis-methacrylamide, a di-epoxide compound, apolyazetidinium compound, a poly(alkylene glycol) diglycidyl ether, orany mixture thereof, and wherein the first polyamidoamine-epihalohydrinresin and the second resin have at least one different property withrespect to one another.
 15. The resin system of claim 14, wherein thesecond resin comprises the urea-formaldehyde resin, and wherein theresin system comprises greater than 30 wt % to about 80 wt % of thesecond resin, based on a combined solids weight of the firstpolyamidoamine-epihalohydrin resin and the second resin.
 16. The resinsystem of claim 14, wherein the first polyamidoamine-epihalohydrin resinhas a pH of about 2 to about 4.5, a charge density of about 2 mEq/g ofsolids to about 4 mEq/g of solids, and a ratio of azetidinium moietiesto amide residues of about 0.4 to about 1.3, and wherein the at leastone different property is a molecular weight, a chemical structure, or acombination thereof.
 17. The resin system of claim 14, wherein thepolyamine has the chemical formula:

wherein R is an alkyl, a hydroxyalkyl, an amine, an amide, an aryl, aheteroaryl, or a cycloalkyl and w is an integer from 1 to about 10,000.18. A method for making a resin system, comprising combining apolyamidoamine-epihalohydrin resin and a urea-formaldehyde resin toproduce the resin system, wherein the resin system comprises greaterthan 30 wt % to about 80 wt % of the urea-formaldehyde resin, based on acombined solids weight of the polyamidoamine-epihalohydrin resin and theurea-formaldehyde resin.
 19. The method of claim 18, wherein: thepolyamidoamine-epihalohydrin resin comprises azetidinium ions and atleast one polyamine that is partially crosslinked with a bridgingmoiety, the bridging moiety is derived from a functionally-symmetriccross-linker comprising a di-acrylate compound, a bis(acrylamide)compound, N,N′-methylene-bis-methacrylamide, a di-epoxide compound, apolyazetidinium compound, a poly(alkylene glycol) diglycidyl ether, orany mixture thereof, the urea-formaldehyde resin has a molar ratio offormaldehyde to urea of about 1:1 to about 3:1 and a weight averagemolecular weight of about 1,000 to about 500,000, and thepolyamidoamine-epihalohydrin resin and the urea-formaldehyde resin arecombined in the presence of a plurality of pulp fibers.
 20. The methodof claim 18, wherein the resin system comprises about 40 wt % to about80 wt % of the urea-formaldehyde resin, based on the combined solidsweight of the polyamidoamine-epihalohydrin resin and theurea-formaldehyde resin.